Constant danger: Scores of police and SWAT team members were surrounding the Boston suburb on Friday morning
Terrorists: Tamerlan Tsarnaev (right) was killed during an exchange of gunfire with police on Wednesday night. His younger brother Dzhokhar (left) is still on the run and reportedly has explosives strapped to his body
On the move: Several different agencies including the Boston police, FBI and SWAT teams were working together
Student: Dzhokhar Tsarnaev graduated from his Cambridge high school and was in college studying medicine
Distress: Police take away a woman near the home of 'suspect 2' Dzhokhar Tsarnaev on Norfolk Street
Stay indoors: A resident views police in tactical gear conduct a search for a suspect in the Boston Marathon bombings
Fast-paced: SWAT teams enter a suburban neighborhood around 9am on Friday to search for the remaining suspect in the Boston Marathon bombings in Watertown, Massachusetts
Escape: A neighbor is escorted to safety as police surround a home while searching for a suspect in the Boston Marathon bombings
Closing in: Police run down a street in Watertown, Massachusetts on Friday screaming at members of the public and media to pull back as the hunt for terror suspect Dzhokar Tsarnaev continues
Lock-down: Police patrol a neighborhood in Watertown as officers go door to door during the manhunt on Friday morning
He had recently been out of the country for six months and said in an interview with a Boston University student magazine in 2010: 'I don't have a single American friend. I don't understand them.'
Tamerlan was once arrested for domestic assault on a girlfriend, ABC
reported.
A friend of the brothers told CNN that he had known them since 2006 and they were 'normal kids' who partied and occasionally smoked. A classmate told CBS that Dzhokhar did not have an accent and that he assumed he had always lived in the U.S.
The uncle of Dzhokhar Tsarnaev urged his nephew to turn himself in. Ruslan Tsarni of Montgomery Village in Maryland, said that the 19-year-old should give himself up and ask for forgiveness.
The father of the suspects said that his son Dzhokhar is a smart and accomplished young man. Anzor Tsarnaev spoke by telephone from the Russian city of Makhachkala on Friday.
Anzor Tsarnaev said: 'My son is a true angel. Dzhokhar is a second-year medical student in the U.S. He is such an intelligent boy. We expected him to come on holidays here.'
Flee: Residents evacuate as police go door-to-door on Norfolk Street in search of a suspect in the Boston Marathon bombings in Cambridge
Door to door: FBI went to every house in the search for Dzhokhar Tsarnaev after he and his brother led police on a car chase on Thursday night
Fears: Onlookers take pictures as they watch from windows while SWAT team members search for one remaining suspect at a neighboring apartment building
To the safety: A police officer evacuates a shoeless man holding a child as members of law enforcement keep the entire area in lock down
Cautious: Police from several different agencies search an apartment complex after a shootout
On his tail: SWAT teams train their weapons toward as house from the roof of a shed as they carry out door-to-door searches for the fugitive terrorist
Dangerous operation: A bomb squad officer uses a robot to remove an item, believed to be a water bottle, from the back of the car allegedly used by the suspects
Flushing him out: SWAT teams along with Boston police flooded into suburban areas of the city during a fast and fluid investigation
On the run: This image provided by the Boston Regional Intelligence Center shows Dzhokhar A Tsarnaev, one of the suspects in the Boston Marathon bombings
Swarming the streets: Dozens of armed officers check houses in the suburb of Watertown after the second suspect fled during a car chase and firefight
Is this their man? This photograph was tweeted by a resident near the shoot-out who claims it is one of the suspects fleeing on foot next to a scorch mark on the pavement
A doctor from Beth Israel Deaconess Medical Center said the terror suspect died after suffering multiple wounds from gunshots and possibly the blast of an explosive.
'There were signs of more than just gunshot wounds, said the doctor, who did not give his name.
'A second suspect was able to flee from that car and there is an active search going on at this point in time,' Colonel Timothy Alben, superintendent of the Massachusetts State Police, told a news conference.
'What we are looking for right now is a suspect consistent with suspect No 2, the white-capped individual who was involved in Monday's bombing of the Boston Marathon,' Alben said.
Police say the two suspects discharged several explosives at police from the vehicle during the pursuit.
According to eyewitness reports, two men believed to be in their early twenties engaged in a furious gun fight with dozens of police on a backstreet of Watertown.
THE CHECHNYA CONNECTION: BOMB SUSPECT BROTHERS LINKED TO ISLAMIC STATE WITH HISTORY OF VIOLENT REBEL ATTACKS
Muslim militants from Chechnya have a long history of unleashing separatist terror attacks on Russia – but the allegations of involvement in the Boston Marathon explosions would mark the first time they have targeted the West.
Buried in the heart of Russia’s Northern Caucasus, the Islamic state has fought against Russian rule for centuries.
It culminated in a bloody and chaotic civil war with the Russian government in 1994 that left tens of thousands dead and the region in ruins. As a result, the area became a hotbed for extremism, and was soon infiltrated by foreign Islamic militants, including those with ties to al Qaeda.
Terrorists have since unleashed a string of attacks on Russian soil and, more recently, abroad.
Russian troops withdrew from Chechnya in 1996 after the first Chechen war, leaving it de-facto independent and largely lawless, but then rolled back three years later following apartment building explosions in Moscow and other cities blamed on the rebels.
Chechnya has stabilized under the steely grip of Kremlin-backed local strongman Ramzan Kadyrov, a former rebel whose forces were accused of massive rights abuses.
But the Islamic insurgency has spread to neighboring provinces, with Dagestan, sandwiched between Chechnya and the Caspian Sea, becoming the epicenter of violence with militants launching daily attacks against police and other authorities.
Militants from Chechnya and neighboring provinces have launched a long series of terror attacks in Russia.
On October 23 2002, over 40, mostly female, terrorists took more than 700 hostages prisoner at a Moscow theatre, demanding an end to the Russian presence in Chechnya. Dressed from head to toe in black hijabs, they became known as The Black Widows.
But when Russian security forces stormed the theatre, guns blazing, the hostage takers responded by detonating homemade bombs strapped to their bodies, killing more than 100 innocent theatre goers.
Then on September 1 2004, a group of 32 heavily-armed, masked men seized control of Middle School Number One and more than 1,000 hostages in Beslan, North Ossetia.
Most of the hostages were children aged from six to sixteen years old.
After a tense two-day standoff, that was beamed around the world, Russian forces raided the building.
A violent, two-hour gunfight followed bringing an end to a siege that ultimately claimed the lives of 331 civilians, 11 commandos and 31 hostage-takers.
The rebels have since claimed responsibility for an array of terrorist attacks, including last year's double suicide bombing of the Moscow subway system that killed 40 people.
In March 2010, two women suicide bombers killed 40 commuters when they blew themselves up on two packed tube trains during the busy rush hour.
And in January a year later, a Chechen suicide bomber unleashed terror on Moscow's Domodedovo Airport when they blew themselves up killing 36 people
In recent years, however, militants in Chechnya, Dagestan and other neighboring provinces have largely refrained from attacks outside the Caucasus.
The allegations of the Caucasus men's role in the Boston's explosions would reinforce long-held claims by Russian officials that insurgents in the Caucasus have been linked to al-Qaeda.
By Matt Blake
Armed and ready: Police officers aim their weapons as they close in on two suspects in the Boston bombings after a university officer at MIT was shot dead on campus
Gun battle: Police officers draw weapons from behind a car during a dramatic shoot-out with two of the suspects in the Boston Marathon bombings
Warning: Boston Police Commissioner Ed Davis tells the media that there is a terrorist on the loose who wants to kill people and tells people not to open their doors
Connections: A map showing the locations of the Boston Marathon bombings, the MIT campus where a police officer was shot dead and the suburb of Watertown where the gunfight broke out
TIMELINE OF TERROR: HOW EVENTS UNFOLDED IN BOSTON
THURSDAY AFTERNOON: FBI released the pictures of the bombing suspects.
MIT CAMPUS, CAMBRIDGE 10.20pm: Police received reports of shots fired on the MIT campus.
10.30pm: An MIT campus police officer was found shot in his vehicle in the area of Vassar and Main streets. According to authorities, the officer was found with multiple gunshot wounds. He was transported to Massachusetts General Hospital and pronounced deceased. Authorities launched an immediate investigation into the circumstances of the shooting. The investigation determined that two males were involved in this shooting.
CAMBRIDGE: 11.30 pm (approx): Police received reports of an armed carjacking by two males in the area of Third Street in Cambridge. The victim was carjacked at gunpoint by two males and was kept in the car with the suspects for approximately a half hour.
Midnight (approx) The victim was released at a gas station on Memorial Drive in Cambridge. He was not injured.
WATERTOWN. Midnight – 1am (approx.) Police immediately began a search for the vehicle and were in pursuit of the vehicle into Watertown. At that time, explosive devices were reportedly thrown from the car by the suspects. The suspects and police also exchanged gunfire in the area of Dexter and Laurel streets. During this pursuit, an MBTA Police officer was seriously injured and transported to the hospital. During the pursuit, one suspect was critically injured and transported to the hospital where he was pronounced deceased. In Watertown, witnesses reported hearing multiple gunshots and explosions at 1am. Dozens of police officers and FBI agents were in the neighborhood and a helicopter circled overhead. 'It sounded like a stick of dynamite went off,' a local resident told the Boston Globe. 'I looked out the window, and it was like nothing I’ve ever seen – blue light after blue light after blue light. '
WATERTOWN 1:30am: The Cambridge bomb squad arrive in Watertown.
CURRENT: An extensive manhunt is ongoing in the Watertown area for the second suspect, who is believed to be armed and dangerous. The case is being investigated by local, state and federal authorities working in cooperation. The Massachusetts State Police Bomb Squad is assessing and removing any potentially explosive devices that may have been thrown on the street in Watertown by the suspects. The investigation remains active and ongoing.
Andrew Kitzenburg spoke to the New York Times about the view he had from his third floor window as he saw two men in jackets shooting at at least 10 police from behind a black Mercedes SUV.
The gun battle was being waged 70 yards apart and Mr. Kitzenburg said that the two men and police were engaged in 'constant gunfire'.
Suddenly a police SUV 'drove towards the shooters' and was fired upon until it was damaged and rolled to the side of the road, crashing into two cars in his driveway.
Dramatically, Mr. Kitzenburg said the two gunmen then threw a large bomb, 'They lit it, still in the middle of the gunfire, and threw it. But it went 20 yards at most.
The unknown device did explode and at that point, one of the men ran towards the gathered police officers and was tackled - it is not clear from Mr. Kitzenburg if he was shot or not.
Then suddenly, the other man returned to his SUV and drove towards police officers and 'put pedal to the metal'.
The car 'went right through the cops, broke right through and continued west.'
Mr Kitzenburg said that the two men had left backpacks by the car and that a police bomb robot was currently dealing with the devices.
Police have told all residents to remain in their homes while they clear the area.
Police screamed at reporters on the scene to turn off phones 'if they want to live' as they hunted for the suspects - fearful that improvised explosives could be remotely detonated.
The shoot-out came hours after the FBI released images and video of the Boston bombing suspects as they launched a hunt for two 'armed and extremely dangerous' men.
The images show them walking in single file toward the finishing line of the race around 13 minutes before the twin detonations.
Hours after the initial photos were made public, investigators released close-up shots this morning of the two suspects that more clearly show the men's faces.
Suspect Number One was wearing a black baseball hat, a dark hooded jacket and khaki pants.
Suspect Number Two was wearing a white basketball cap on backwards and a dark hooded sweatshirt.
The FBI made no mention of the men's height, weight or age range and would not discuss the men's ethnicity.
'It would be inappropriate to comment on the ethnicity of the men because it could lead people down the wrong path potentially,' said FBI agent Greg Comcowich, a spokesman for the Boston FBI office.
The information on the first suspect was developed within a day or so before its release, DesLauriers said.
Agent Daniel Curtin said the FBI did not issue the photos earlier because authorities wanted to be meticulous: 'It's important to get it right.'
And according to FBI sources who have spoken to CNN, still unreleased footage reveals that the suspects stayed at the scene to watch the devastation the two blasts created.
'When the bombs blow up, when most people are running away and victims were lying on the ground, the two suspects walk away pretty casually,' said the official to CNN, who has seen the unreleased video.
'They acted differently than everyone else,' he added.
VIDEO Lay down. Show me your hands. Mobile footage claims to show gun battle
Hunt: Police officers with their weapons out search for a suspect on the streets of Watertown
Alert: A police officers rides in the back of a van with his weapons out following a tense night of police activity
Link: It's not clear whether the incident which unfolded tonight is related to the Boston bombing on Monday
False alarm: A man lies spread out on the ground as police trains their weapons on him, however it is thought he is not of the suspects
Shoot out: According to eyewitness reports, two men believed to be in their early twenties engaged in a furious gun fight with dozens of police on a backstreet of Watertown
Massachusetts State Police spokesperson Dave Procopio said that they believe multiple possible explosive devices were used against police tonight during this incident at Watertown
Witnesses reported hearing explosions and gunfire as the police apparently engaged the two men and local television footage showed one man lying spread out on the ground, with his arms out, surrounded by police.
However, it is now thought that this man is not one of the suspects wanted by law enforcement.
Police officials would not discuss the incident with reporters.
When asked for information on the incident, Boston police spokeswoman Neva Coakley frantically told MailOnline, 'I don't know anything... I don't know anything at this point.'
An officer at the Waterdown police department responded similarly to questions about the incident, saying only, 'They are out in the street, that's all I know,' before hanging up the phone.
Stand-off: A police officer trains his weapon as chaos descends on Boston after multiple shots and explosions were heard in the suburb of Watertown
As the battle raged, police urged everyone to stay inside of their homes. The area is still considered extremely dangerous.
Massachusetts state police spokesman Frank Schifone told MailOnline that only one suspect is accounted for and he’s in the hospital.
Asked whether there is any tie between tonight’s incidents and the marathon bombings, he said, ‘At this time we don’t know. It’s all unfolding.’
Initial reports suggest that one suspect is in custody and a police officer has been injured.
Earlier, Cambridge police and the Middlesex District Attorney's office confirmed an officer was shot dead responding to a report of a disturbance when he was fired upon multiple times.
He later died at a hospital. His name was not immediately released.
State police spokesman Dave Procopio says the shooting took place about 10:30 p.m. outside an MIT building.
Procopio says authorities are searching for a suspect or suspects. No arrests have been made.
MIT police, Cambridge police and state police are involved in the investigation.
About 11,000 people attend the prestigious school.
The campus website said police were sweeping the campus and urged people to stay indoors.
In a statement issued to the press, law enforcement officials said that no arrests have been made in the shooting and the search for a suspect or suspects is ongoing.
Massachusetts Institute of Technology (MIT) in Boston issued an emergency alert at 10.48 pm on Thursday night reporting that shots had been fired putting an officer was down and that a shooter was on the run. It has been reported that a 'Hispanic looking man' took the officer's weapon from him before firing on him. MIT posted this alert to their website
'At 10:48 PM today gunshots were reported near Building 32 (Stata) which is currently surrounded by responding agencies.
'The area is cordoned off. Please stay clear of area until further notice. Unknown if injuries have occurred.. Although the situation is considered active and extremely dangerous, an investigation is underway.
'Updates will be provided at this site when more information becomes available.'
The injured officer was from MIT Police and was being taken to Mass. General Hospital where he is currently in a critical condition.
The campus newspaper 'The Tech' posted to Twitter, 'Shots fired near 32 Vassar St (Stata Center), police officer down. Please stay inside.'
Massachusetts State Police initially said the officer has suffered ‘life-threatening injuries.’ He was transported to Massachusetts General Hospital.
Clearer shot: David Green, 49, captured this picture with his iPhone likely of the man identified by the FBI as Suspect Number Two, as Green faced east on the corner of Fairfield and Boylston Streets, shortly after the Boston Marathon blasts
Comparison: Suspect Number Two is seen in surveillance footage from the FBI released on Thursday (left). The man closely resembling that suspect is seen (right) in a clearer image released by a man who had competed in the Boston Marathon and captured the picture as he ran toward the site of the blast
Earlier on Thursday, FBI Special Agent Richard DesLauriers spoke at a press conference to release photos of two men identified as 'suspect one' and 'suspect two' from surveillance footage near the blast site.
Suspect one is dressed in dark clothes, a baseball cap and wearing sunglasses, while suspect two is clearly wearing a white baseball cap on backwards - both are seen in the images with back packs on.
Originally, Mr Green thought that his photograph also included the man referred to as Suspect Number One, who was wearing a black baseball hat, a dark hooded jacket and khaki pants.
Though there is a man wearing khaki pants in Mr Green's picture, who is seen next to a woman in a pink jacket, the man in his picture is wearing a red jacket and is not believed to be the suspect being hunted down by the FBI.
The District Attorney's office in Boston later confirmed the officer had passed away. The school is describing the situation as 'active and extremely dangerous.' - and FBI agents have been witnessed at the scene.
Boston has been on edge after two bombs ripped through the crowd near the finish line of the Boston Marathon, killing three people and injured 183 more.
The image taken by Florida businessman David Green emerged just hours after the FBI lodged an appeal to identify two suspects behind the attacks that killed three and injured over 180.
His picture shows runners and spectators fleeing in panic around the suspect, closely resembling the man identified by officials as Suspect Number Two, who is wearing a white basketball cap and a dark hooded sweatshirt.
that agents believe the man is Suspect Number Two.
Earlier on Thursday, the FBI launched a hunt for two 'armed and extremely dangerous' men now considered suspects in Monday's deadly Boston bombings - as it was revealed they stayed to watch the carnage unleashed by the twin blasts.
The earlier images, released by the FBI, show the as-yet unidentified men walking in single file toward the finishing line of the race eastward along Boylston Street at 2:37pm, approximately 13 minutes before the twin detonations which came 12 seconds apart.
Hours after the initial photos were made public, investigators released closeup shots on Friday morning of the two suspects that more clearly show the men's faces.
This combination of Associated Press file images released by the FBI on Thursday show two images taken from surveillance video of what the FBI are calling suspect number 1, (left), in black cap,and suspect number 2, (right), in a white cap
Mr Green captured the astonishingly clear photo with his iPhone at 2:50pm on Monday, as he ran toward the site of the first blast.
The married 49-year-old, who is the CEO of an athletic apparel company, had competed in the marathon and finished the race at around 1:40pm.
It was the first time he had competed in the race.
He was in search of friends who were cheering for him in the crowd when the blasts occurred, he told CNN's Piers Morgan on Thursday night.
As he ran toward the scene of the blast, he captured just one picture with remarkable clarity.
After the explosions, Mr Green posted the photo to his Facebook page and also contacted the FBI on Monday, alerting them to the scene he had captured on his smartphone.
On Thursday, after officials released the photos of the main suspects in the Boston bombings, one of Mr Green's friends contacted him and noted the resemblance to the man in Green's photograph and the picture of Suspect Number Two released by the FBI.
'I have spoken to several agents,' Mr Green, who is from Jacksonville Beach, Fla. told the New York Times.
'They have got the photos, they are picking up material from me, and I don’t think there's any doubt,' he said.
During his interview with CNN, Mr Morgan also probed about the authenticity of the photo.
FBI officials 'told me it looks like a good picture,' he said.
After Mr Green's photograph was published, a federal investigator in Boston told the MailOnline that officials would not comment on the credibility of photographs other than the earlier pictures released by the FBI.
The Florida resident told Piers Morgan that he is still numb from his experience in New England earlier this week.
'I was incapable of helping anybody. It was well beyond anything I had seen or knew how to deal with,' he said.
'It all happened very fast, there was a lot of screaming and a lot of noise.'
As he sorts through the emotional impact of the experience he said it brings him some relief knowing that a simple photograph could possibly help investigators locate the perpetrators of the horrific attack.
'I hope if it helps solve this case, [it could] in a way redeem what I couldn't do at the site,' he told the CNN host.
In the photo taken by Mr Green, the man who appears to be Suspect Number Two is not wearing a backpack in the aftermath of the blast. He noted that a closeup of the photograph shows how the side of the suspect's hat has a number 3 emblazoned on the side.
Previously, commentators had speculated it appeared that the number on the hat could have been a 7.
Agent DesLauriers confirmed there are images of suspect two placing a backpack on the floor at the site of the second explosion 'within minutes' of the blast - but the FBI declined to show this sequence as specific details about it may be important for future questioning.
Making a direct appeal to the public, agent DesLauriers asked for any information, however insignificant that could lead to them identifying and eventually speaking to these individuals who are wanted in connection with the terror attack which killed three and injured over 180 people.
DesLauriers said investigators were particularly interested in interviewing witnesses who were in front of the Forum restaurant, site of the second blast.
He also cautioned the public not to approach the two men, even if they think they identify them.
'We consider them to be armed and extremely dangerous, No one should approach them, do not apprehend them,' said DesLauriers.
'Do not take any action on your own. If you see these men, contact law enforcement.'
The bombings that killed three people and wounded over 180 began a week of security scares that rattled the United States and evoked memories of the September 11, 2001 hijacked plane attacks.
Did the suspects stay to watch what they had done:
Both men carried backpacks that were believed to contain the bombs. The man identified as suspect number one wore a dark baseball cap. suspect number two wore a white cap backwards and was seen setting down his backpack on the ground, agent DesLauriers said.
According to FBI sources who have spoken to CNN, still unreleased footage reveals that the suspects stayed at the scene to watch the devastation the two blasts created.
'When the bombs blow up, when most people are running away and victims were lying on the ground, the two suspects walk away pretty casually,' said the official to CNN, who has seen the unreleased video.
'They acted differently than everyone else,' he added.
What is the Significance of what the Two Men were Wearing - Internet pours over their attire
Internet users immediately began scrutinizing the mens' clothing and backpacks on Reddit, and within minutes, one user had identified the cap on one of the suspects as a Bridgestone Golf hat, available at Walmart.com for $14.99.
The black hat has distinct white markings and a logo above the brim that looks identical to the Bridgestone hat.
The hat on the second suspect has proven more difficult to identify.
The suspect wore the white cap backwards in the images and the logo on the front is only visible for a split second on a grainy video.
There appears to be a number - possible a '7' - on the side of the hat.
Some on Reddit suggested that it could be a Polo Ralph Lauren Hat - the markings are similar to one that's available online for around $30 - but that hat only comes with the number '3' on the side.
'Today we are enlisting the public's help to identify the two suspects,' Richard DesLauriers, the U.S. Federal Bureau of Investigation's special agent in charge in Boston, told a news conference.
'Somebody out there knows these individuals as friends, neighbors, co-workers or family members of the suspects. Though it may be difficult, the nation is counting on those with information to come forward and provide it to us,' said DesLauriers while cautioning that they were considered armed and dangerous.
Investigators hoped the men would be identifiable within hours of the release of the pictures and video, a national security official said, speaking on condition of anonymity.
Law enforcement were looking at the men for some period of time before deciding to make the videos public, and they had extensive video and still pictures to justify the FBI decision to label the two men as suspects, the official said.
This image released by the FBI on Thursday, April 18, 2013, shows in a image from video what the FBI are calling 'Suspect Number One', front left, in black cap, and 'Suspect Number 2, in white cap, (back right)
Mingling with the thousands of spectators: How the two men made their way through the crowd raising no suspicion.
Investigators were looking at the men for some period of time before deciding to make the videos public, and they had extensive video and still pictures to justify the FBI decision to label the two men as suspects one official said.
But through intensive viewings, the FBI concluded that the two men were associated with each - as they strolled calmly through the crowd.
The two men are seen walking together in an eastwardly direction at the intersection of Boylston and Gloucester Streets within proximity of the finish line at 14.37 - exactly 13-minutes before the twin blasts at the marathon.
Suspect one, dressed in a black baseball cap and khaki-colored trousers is around five paces ahead of suspect two, who is wearing a white baseball cap backwards, with a grey hoody underneath a black coat black pants.
In grabs taken from video, the two suspects, who appear to be in the early to mid-twenties, do not talk to each other, but move in the same direction with backpacks hanging off their shoulders.
The footage shows suspect two place down his backpack at the scene of the second explosion just in front of the Forum restaraunt and then move back down Boylston Street in a westward direction.
Suspect one is seen moving through the marathon crowd toward the finish line and scene of the first blast - without suspect two who has disappeared from police view.
Video of suspect two planting his device exists, but the FBI has chosen not to release it because were the media to repeatedly show the suspects leaving the bomb, it might cause some people to overreact if they came into contact with them.
In the new images released by the FBI - the as yet-unidentified men known as suspect one and suspect two - are first seen walking in single file along Boylston Street after turning at the intersection of Gloucester. Suspect number two wearing a white hat was seen setting down a backpack at the site of the second explosion at the Forum restaurant
Was a timer used or were the blasts triggered using a cell phone?
In the hours since the photographs were released, speculation has centered on one of the images which reportedly shows suspect two with a cell phone to his ear.
ABC News quoted authorities as saying that they believe a mobile phone device was used to trigger one of the Boston marathon explosions.
A senior law enforcement official said that a 'Massive effort was underway to capture every cell phone call in that local area at the time of the bombings.
And that 'Attempting to trace the call back to the bomber is a time consuming process, but a top top priority.'
However, a law enforcement source told CNN that despite a belief that the bombs were detonated by remote control, the bombs were probably detonated by timers.
However, The FBI has said details of the detonating system are still unknown.
How the Boston bombing victim in iconic photo helped identify attackers
Jeff Bauman, whose legs were blown away in the first explosion, told investigators that he stared right into the eyes of the man who tried to kill him.
Bauman lost both his legs below the kneww in the blast and a picture of him in a wheelchair, pale and bloodied was broadcast around the world.
But just before 2.50 p.m. on Monday, Bauman was waiting in the crowd for his girlfriend to cross the finish line when a man wearing a cap, sunglasses and a black jacket over a hooded sweatshirt looked at Jeff, 27, and dropped a bag at his feet.
Two and a half minutes later the bag exploded - destroying Jeff's legs - but Jeff had seen a man who matched suspect one's description.
'He woke up under so much drugs, asked for a paper and pen and wrote, ‘bag, saw the guy, looked right at me,’ Chris Bauman said yesterday in an interview given to Bloomberg.
However, despite his severely medicated state, Jeff managed to give an interview and description to the FBI - which helped them narrow down the man they were looking for.
At least one other person of interest who featured in crime scene pictures had been ruled out as a suspect. Also ruled out earlier in the week was a Saudi student who was injured in the attacks, the official said.
The images were released hours after President Barack Obama and first lady Michelle Obama attended an interfaith service at a Roman Catholic cathedral in Boston to remember the three people killed and more than 180 wounded in the twin blasts Monday at the marathon finish line.
The break in the investigation came just days after the attack that tore off limbs, shattered windows and raised the specter of another terrorist attack on U.S. soil. FBI photo-analysis specialists have been analyzing a mountain of surveillance footage and amateur pictures and video for clues to who carried out the attack and why.
FBI Special Agent in Charge Richard DesLauriers and JTTF officials unveil images depicting two suspects in their investigation in Boston, Massachusetts, in a dramatic press conference on Thursday afternoon
Map showing the Boston crime scene perimeter where experts are trying to piece together the clues left behind the deadly marathon explosion
Generally, law enforcement agencies release photos of suspects only as a last resort, when they need the public's help in identifying or capturing someone.
Releasing photos can be a mixed bag: It can tip off a suspect and deny police the element of surprise. It can also trigger an avalanche of tips, forcing police to waste valuable time chasing them down.
Within moments of the announcement, the FBI website crashed, perhaps because of a crush of visitors.
In the images, both men appear to be wearing dark jackets. Suspect 1 appears to be wearing a backpack. The planting of the backpack is not depicted in the video footage that was made public.
The FBI made no mention of the men's height, weight or age range and would not discuss the men's ethnicity.
'It would be inappropriate to comment on the ethnicity of the men because it could lead people down the wrong path potentially,' said FBI agent Greg Comcowich, a spokesman for the Boston FBI office.
The information on the first suspect was developed within a day or so before its release, DesLauriers said. Agent Daniel Curtin said the FBI did not issue the photos earlier because authorities wanted to be meticulous: 'It's important to get it right.'
At the Cathedral of the Holy Cross earlier in the day, Obama declared to the people of Boston: 'Your resolve is the greatest rebuke to whoever committed this heinous act.' He spoke in almost mocking terms of those who commit such violence.
'We finish the race, and we do that because of who we are,' the president said to applause. 'And that's what the perpetrators of such senseless violence - these small, stunted individuals who would destroy instead of build and think somehow that makes them important - that's what they don't understand.'
'We will find you,' he warned those behind the attack.
Seven victims remained in critical condition. Killed were 8-year-old Martin Richard of Boston, 29-year-old restaurant manager Krystle Campbell of Medford, Mass., and Lu Lingzi, a 23-year-old Boston University graduate student from China.
Horrific Scene: The aftermath of the Boston Marathon bombing on Monday evening in the city
Video and photos recovered in the investigation are being examined and enhanced by an FBI unit called the Operational Technologies Division, said Joe DiZinno, former director of the FBI lab in Quantico, Va.
Investigators are looking at video frame by frame - a laborious process, though one aided by far more sophisticated facial recognition technology than is commercially available, forensic specialists said.
'When you have something that is this high-profile, they are going to use every available resource that they have,' said former Miami federal prosecutor Melissa Damian Visconti.
The investigation will probably collect about a million hours of videotape from fixed security cameras and cellphones and cameras used by spectators, said Gene Grindstaff, a scientist at Intergraph Corp., a Huntsville, Ala., company that makes video analysis software used by the FBI and other law enforcement agencies.
But after years of investigating terrorist incidents and other crimes, the FBI is practiced at cataloging, categorizing and analyzing such evidence and will winnow it down dramatically, he said.
'Back in the days of 20 years ago, you were lucky if you had video and it was probably of poor quality and it took a tremendous amount of enhancement. Today you have a completely different issue,' Grindstaff said.
Injured people and debris lie on the sidewalk near the Boston Marathon finish line following an explosion in Boston, Monday, April 15, 2013
Investigators can set the video analysis software so that it automatically searches for certain types of objects or people matching a height and weight description. The software can also spot patterns that analysts might not notice, such as a certain car that turns up in different places, Grindstaff said.
DiZinno, who ran the FBI lab from 2007 to 2010, said any retrieved bomb components such as the pressure cookers, shrapnel and pieces of timers or wire will be closely examined for fingerprints, DNA, hairs and fibers.
The bomb components would be traced by figuring out the item's maker, where each piece is typically purchased and whether the device resembles any bombs the FBI has seen in past attacks. The FBI lab keeps a detailed file on past bombings, including many overseas attacks.
'Let's say there was a timer,' DiZinno said. 'Was there a serial number? Who was the manufacturer? That can provide leads for investigators.'
The press conference held at 5pm on Thursday was the first time in over 24 hours that federal officials released any information about the case.
Minutes before the long-awaited briefing, the local Boston FOX affiliate released a picture of a person that they believe is the possible second bomber.
The station said that they confirmed the photo was confirmed by the FBI.
Earlier on Thursday, high school student Salah Barhoum denied being involved in the bombing after much speculation about images of the 17-year-old track star standing next to his reported coach, who was wearing a white baseball cap.
Barhoum spoke to MailOnline, saying that as soon as he saw his picture circulating online, he went to speak to police on his own accord and was cleared of any charges relating to the fatal bombing that killed three people and injured more than 180 others.
Since late Wednesday night, no further clarification was announced about the on-going investigation in spite of multiple reports that police had clear images of two potential suspects.
There was no reason given for their reticence in handing the picture out to media outlets throughout the day on Thursday.
According to CBS News senior correspondent John Miller the big debate right now among law enforcement is whether to release the images of the men to the public.
'It's been a tough call,' said Miller. 'For investigators, there's always a difficult choice because if the person doesn't know you're looking for him, he may stay in place, you may catch up to him,' said Miller.
'If he does know you're looking for him, he may run. On the other hand, if you don't get him, it's always great to enlist 20 million or so more eyes in the public who may be able to give you a location right away.
'That's not going to happen tonight anymore. It was going to happen earlier- they've rethought that. It may happen with the release of that picture tomorrow. They're reassessing.'
At this point, investigators have been sifting through masses of images, gathered by either civilian fan footage and multiple surveillance cameras in the area.
Anyone with any information regarding the two suspects is urged to call the FBI on 1800 222 5324
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