Medical responders run an injured man past the finish line the 2013 Boston Marathon following an explosion.

A woman is comforted by a man near a triage tent set up for the Boston Marathon after explosions went off.

Police evacuate the scene after the explosions hit near the finish line of the Boston Marathon.

Racers and race officials look for loved ones after multiple explosions near the finish line of the Boston Marathon.

Still image taken from video courtesy of NBC shows an explosion at the Boston Marathon.

An injured man stands at the scene of the Boston Marathon explosions.

A victim is attended to at the scene of the Boston Marathon bombings.

Injured people are attended to at the scene of the Boston Marathon explosions.

Blood can be seen on the footpaths as men in bomb-disposal suits investigate the site of the Boston Marathon explosions.

An injured woman is attended to at the scene of the Boston Marathon terrorist attack.

A handcuffed suspect is surrounded by police officers about a mile from the Boston Marathon finish line. Police are holding a person in custody in relation to the attacks.

An injured child is carried away from the blasts.

A child is comforted after explosions went off at the 117th Boston Marathon.

Police officers and bomb-detecting dogs investigate after explosions near the finish line of the Boston Marathon.

Investigators survey the site of a bomb blast on Boylston St, Boston, a day after two explosions hit the Boston Marathon.

Boston Marathon volunteers embrace as they wait for a memorial service to start at the Cathedral of the Holy Cross.

Boston Police officers on bikes patrol outside the Cathedral of the Holy Cross, where President Barack Obama was set to attend an interfaith memorial service.

President Barack Obama and first lady Michelle Obama at an interfaith memorial service for victims of the Boston Marathon bombings at the Cathedral of the Holy Cross in Boston.

The name of one of the Boston Marathon bombing suspects, Tamerlan Tsarnaev, was listed on the US government’s highly classified central database of people it views as potential terrorists. But the list is so vast that this did not mean authorities automatically kept close tabs on him, sources close to the bombing investigation said.

Tsarnaev, 26, was killed in a police shootout Friday, while his younger brother Dzhokhar, 19, was later captured.

Dzhokhar has admitted to FBI agents that he and his dead brother Tamerlan carried out the deadly bombings, The Boston Globe reports.

1 of 6 FBI The two suspects for the Boston Marathon bombings. 'Suspect 1' in front, 'suspect 2' in rear. 2 of 6 Boston Marathon bombings 'suspect 1'. 3 of 6 FBI Boston Marathon bombings 'suspect 2'. 4 of 6 FBI Boston Marathon bombings 'suspect 1'. 5 of 6 FBI Boston Marathon bombings 'suspect 2'. 6 of 6 FBI Boston Marathon bombings 'suspect 2'.

The 19-year-old, who remains hospitalised for wounds suffered in a shootout with police, also said that he and his brother killed an MIT campus policeman last week.

The surviving suspect made his statements at Beth Israel Deaconess Medical Center. His health is apparently improving - the hospital listed his condition as fair.

Dzhokhar was formally charged this week with using weapons of mass destruction against people in detonating two bombs on April 15 near the finish line of the Boston Marathon, killing three people and injured more than 200 others.

1 of 26 The Massachusetts State Police released this image taken from a police helicopter as police surrounded the marathon bombing suspect. 2 of 26 The Massachusetts State Police released this image taken from a police helicopter as police surrounded the marathon bombing suspect. 3 of 26 Twitter This photo is allegedly of the suspect Dzhokhar Tsarnaev laying face up on the ground after his capture. International media are working on verifying the image. 4 of 26 Law enforcement officials depart the search area in Watertown for Dzhokar Tsarnaev after the stay at home alert has been lifted. 5 of 26 Residents are asked to leave their home as SWAT teams conduct a house to house search for Dzhokar Tsarnaev, the one remaining suspect in the Boston Marathon bombing. 6 of 26 A girl looks out her window as law enforcement searches for Dzhokar Tsarnaev, the remaining suspect in the Boston Marathon bombings, in Watertown, Massachusetts. 7 of 26 A photo from the profile of Dzhokhar Tsarnaev taken from social networking site VK. 8 of 26 Anzor Tsarnaev, who calls himself father of Dzhokhar and Tamerlan Tsarnaev, the two suspects in the Boston Marathon bombings, gives an interview in Makhachkala in which he claims his sons have been framed. 9 of 26 MIT Patrol Officer Sean A. Collier, 26, of Somerville, Massachusetts is pictured in this undated MIT handout photo. Collier was shot April 18, 2013 on the MIT campus following an altercation with the suspects in the Boston Marathon bombing, according to media reports. He was transported to Massachusetts General Hospital, where he was pronounced dead. 10 of 26 Massachusetts Bay Transportation Authority (MBTA) police officer Richard Donohue Jr., is shown in this handout photo provide by the MBTA April 19, 2013. Donahue was injured in a shootout late April 18, 2013 with Boston Marathon bombing suspects Tamerlan and Dzhokhar Tsarnaev. 11 of 26 Reuters A member of the SWAT team trains a gun on an apartment building during a search for the remaining suspect in the Boston Marathon bombings. 12 of 26 Reuters SWAT teams enter a suburban neighborhood to search for the remaining suspect in the Boston Marathon bombings in Watertown. 13 of 26 Reuters A bomb technician walks away after preparing the controlled detonation of a suspicious object during a search for a suspect in the Boston Marathon bombing. 14 of 26 Reuters SWAT teams enter a suburban neighborhood to search for the remaining suspect in the Boston Marathon bombings in Watertown. 15 of 26 Reuters Members of the SWAT team cover an apartment during the search for the remaining suspect in the Boston Marathon bombings. 16 of 26 Reuters SWAT teams enter a suburban neighborhood to search for the remaining suspect in the Boston Marathon bombings in Watertown. 17 of 26 Boston Poilice NEW IMAGE: Police have released this new photo showing the man believed to be a suspect in the Boston Marathon bombing, taken at a 7-Eleven store in Cambridge, just across the river from Boston. 18 of 26 Reuters Superintendent and Colonel of the Massachusetts State Police Timothy Alben speaks to reporters about the status of the two suspects in the Boston Marathon bombings in Watertown. 19 of 26 Reuters A law enforcement vehicle carries a bomb disposal device through Watertown. 20 of 26 Reuters Law enforcement officials during a search for the two men suspected of setting off two explosions during the Boston Marathon. 21 of 26 Suspects wanted for questioning in relation to the Boston Marathon bombing. 22 of 26 Suspects wanted for questioning in relation to the Boston Marathon bombing. 23 of 26 Reuters Police officers keep a man on the ground in Watertown, Massachusetts. 24 of 26 Reuters Law enforcement officers talk at the scene of a police manhunt in Watertown, Massachusetts. 25 of 26 Reuters Police officers keep a man on the ground in Watertown, Massachusetts. 26 of 26 Reuters Heavy armed police officers arrive at the scene of a manhunt in Watertown, Massachusetts.

Sources today said older brother Tamerlan’s details were entered into TIDE, a database maintained by the National Counterterrorism Center, because the FBI spoke to him in 2011 while investigating a Russian tip-off that he had become a follower of radical Islamists.

The FBI found nothing to suggest he was an active threat, but all the same placed his name on the ‘Terrorist Identities Datamart Environment’ list. The FBI has not said what it did find about Tsarnaev.

But the database, which held more than half a million names, was only a repository of information on people who US authorities saw as known, suspected or potential terrorists from around the world.

Reuters CHARGED: Boston bombings suspect Dzhokhar Tsarnaev.

Because of its huge size, US investigators did not routinely monitor everyone registered there, said US officials familiar with the database.

As of 2008, TIDE contained more than 540,000 names, although they represented about 450,000 actual people, because some of the entries are aliases or different name spellings for the same person.

Fewer than five per cent of the TIDE entries were US citizens or legal residents, according to a description of the database on the NCTC website.

SUSPECTED BOMBER: Tamerlan Tsarnaev allegedly read jihadist websites and extremist propaganda in the run-in to the Boston Marathon bombings.

TIDE AND OTHER DATABASES

The TIDE database is one of many federal security databases set up after the September 11, 2001, attacks. The database system has been criticised in the past for being too cumbersome, especially in light of an attempted attack on a plane in 2009.

Intelligence and security agencies acknowledged in Congress that they had missed clues to the Detroit ‘‘underpants bomber’’ Umar Farouk Abdulmutallab.

Officials said after the incident that he had been listed in the TIDE database.

Republican Senator Susan Collins said there were problems in sharing information ahead of the Boston bombings, too.

‘‘This is troubling to me that this many years after the attacks on our country in 2001 that we still seem to have stovepipes that prevent information from being shared effectively,’’ she said.

Collins was speaking after the FBI gave a closed-door briefing to the Senate Intelligence Committee today, but she did not elaborate.

However, in the case of Tamerlan Tsarnaev, the issue appeared to be that because he was not deemed an active threat, his name was only briefly on a list that would have triggered monitoring.

Tsarnaev was not put on the ‘‘no-fly’’ list that would have banned him from boarding an airplane in the United States. Neither was he named on the Selectee List, which would have required him to be given extra security screening at airports.

Another list, the Terrorist Screening Database, is a declassified version of the highly classified TIDE with fewer details about terrorist suspects. One source said Tsarnaev was on this list, too.

After being put in the TIDE system, his name was entered in another database, this one maintained by the Homeland Security Department’s Customs and Border Protection bureau which is used to screen people crossing US land borders and entering at airports or by sea.

Tsarnaev was flagged on that database when he left the United States for Russia in January 2012 but no alarm was raised, presumably because the FBI had not identified him as a threat after the interview.

When he returned from Russia six months later, he had already been automatically downgraded in the border database because there was no new information that required him to continue to get extra attention. So he did not get secondary inspection on his re-entry at New York’s JFK Airport. It was unclear exactly what the procedure was for such a downgrade.

Sean Joyce, deputy director of the FBI, defended the FBI’s performance in the Boston bombings at two closed hearings in Congress on Tuesday.

While government agencies declined to publicly discuss how the watch list system handled Tsarnaev, Homeland Security Secretary Janet Napolitano disclosed some details at a separate, open hearing on immigration on Capitol Hill.

‘‘Yes, the system pinged when he was leaving the United States. By the time he returned, all investigations — the matter had been closed,’’ Napolitano told the Senate Judiciary Committee.

Bob Grenier, former chief of the CIA’s Counterterrorism Center, said the effectiveness of watch list systems is dependent on what information was put in them, adding that unless authorities had a strong piece of information against somebody, they were not going to put restrictions on people in a free society.

Republican Senator Lindsey Graham said law enforcement should have kept a closer eye on Tsarnaev after the FBI spoke to him two years ago. The FBI should also have realised last week following the bombings that he was in databases, Graham told reporters.

‘‘After the bomb went off, don’t you think one of the first things the FBI would do is say, ’Have we interviewed anybody in the Boston area that may fit the profile of doing this?’ How could his name not pop up, the older brother? And when you have the photo the whole world is looking at, how could we not match that photo with him already being in the system?’’ Graham said.

RADICAL DRIVERS

Dzhokhar said he and his brother, a pair of Chechens who had lived in the US for more than a decade, were driven to the attack by jihadist radicalism sparked by the US wars in Iraq and Afghanistan in which thousands of Muslims have died, a federal law enforcement official said.

The 19-year-old said they learned how to make the pressure cooker bombs used in the attack from an al-Qaeda website, said the official.

Tamerlan is believed to have instigated the attack after turning devoutly religious and possibly reading radical Islamic dogma on internet sites or associating with radicals during visits to Russia, law enforcement officials said.

Relatives have suggested in news interviews that he would have held considerable sway over his younger brother, a college student.

Officials do not believe Dzhokhar and his brother were linked to any organised militant groups, media report.

No evidence has surfaced so far that the two Chechen brothers were influenced by a foreign terrorist organisation.

But FBI agents will be conducting a worldwide investigation to examine the origins of their apparent radicalisation, and to determine for certain whether they acted alone, were assisted by other conspirators or received any assistance along the way, the official said.

The investigation also will examine whether they were involved in an unsolved triple murder, apparently carried out on the 10th anniversary of the September 11, 2001, terror attacks, in Waltham, Massachusetts, the law enforcement official said.

A friend of Tamerlan, 25-year-old Brendan Mess, was among three young men whose throats were slit, apparently a day earlier than investigators originally thought, according to The Boston Globe.

MENTOR 'TOOK HIS BRAIN'

In the years before the bombings, Tamerlan fell under the influence of a new friend, a Muslim convert who steered the religiously apathetic young man toward a strict strain of Islam.

Under the tutelage of a friend known to the Tsarnaev family only as Misha, Tamerlan gave up boxing and stopped studying music, his family said. He began opposing the wars in Afghanistan and Iraq.

He turned to websites and literature claiming that the CIA was behind the September 11th attacks and Jews controlled the world.

"Somehow, he just took his brain," said Tamerlan's uncle, Ruslan Tsarni, who recalled conversations with Tamerlan's worried father about Misha's influence.

Throughout his religious makeover, Tamerlan maintained a strong influence over his siblings, including Dzhokhar.

"They all loved Tamerlan. He was the eldest one and he, in many ways, was the role model for his sisters and his brother," said Elmirza Khozhugov, 26, the ex-husband of Tamerlan's sister, Ailina.

"You could always hear his younger brother and sisters say, 'Tamerlan said this,' and 'Tamerlan said that.' Dzhokhar loved him. He would do whatever Tamerlan would say.

"Even my ex-wife loved him so much and respected him so much," Khozhugov said. "I'd have arguments with her and if Tamerlan took my side, she would agree: 'OK, if Tamerlan said it.'"

Khozhugov said he was close to Tamerlan when he was married and they kept in touch for a while but drifted apart in the past two years or so.

"Of course I was shocked and surprised that he was Suspect No 1," Khozhugov said, recalling the days after the bombing when the FBI identified Tamerlan as the primary suspect.

"But after a few hours of thinking about it, I thought it could be possible that he did it."

- Agencies