So what's the point of a terror list?
"Boston Bombing Suspect On Terror List In 2011" Yahoo News
The name of the dead Boston bombing suspect was placed on a federal government terrorist database 18 months before the marathon attack, according to US officials.
Sources close to the investigation said that the Russian-born Chechen brothers Tamerlan and Dzhokhar Tsarnaev had been radicalised through jihadist materials on the internet.
According to the US officials Tamerlan was added by the CIA to a classified database of known and suspected terrorists - the Terrorist Identities Datamart Environment, known as Tide.
The 26-year-old amateur boxer, who according to his aunt had become a devout Muslim in recent years, had been investigated following a request from Russia's security service, the FSB, which had concerns he had become a follower of radical Islam.
In March 2011, about six months earlier, the FBI had separately investigated Tamerlan, also at Russia's request, but the FBI found no ties to terrorism, officials said.
However, the spelling the FBI used in its investigation was not the same as the spelling used by the CIA.
His six-month trip to Russia in January 2012 triggered no alerts because the FBI had closed its case and Tamerlan was not considered a risk.
According to the officials Dzhokhar, 19, who is being questioned in hospital but unable to speak due to a reportedly self-inflicted throat injury, has told investigators his brother had only recently recruited him to be part of the attack.
The revelations will intensify concerns about failings of the security services in both investigating and identifying Tamerlan as a terrorist threat.
Shortly after the attacks which killed three, including an eight-year-old boy and injured more than 180, US officials said the intelligence community had no information about threats to the marathon before the April 15 explosions.
Tamerlan, whom authorities have described as the driving force behind the plot, was killed in a shoot-out with police.
US officials are expected to brief the Senate on the investigation today but the latest disclosures are expected to prompt an inquiry into security failures.
The Russian President, Vladimir Putin, said in his annual question and answer session today that the bombings illustrated the need for Russia and the US to work more closely on security matters.
He said: "If we truly join our efforts, we will not allow these strikes and suffer such losses."
Officials said Wednesday that Dzhokhar acknowledged to the FBI his role in the attacks but did so before he was advised of his constitutional rights to keep quiet and seek a lawyer.
It is unclear whether those statements would be admissible in a criminal trial and, if not, whether prosecutors even need them to win a conviction.
Authorities have said Dzhokhar exchanged gunfire with them for more than an hour Friday night before they captured him inside a boat in a suburban Boston backyard.
Dzhokhar has told the FBI that he and his brother were angry about the wars in Afghanistan and Iraq and the killing of Muslims there, officials said.
How much of those conversations will end up in court is unclear. The FBI normally tells suspects they have the right to remain silent before questioning them so all their statements can be used against them.
Investigators have found pieces of remote-control equipment among the debris from the bombing and were analysing them, officials said.
One official described the detonator as "close-controlled," meaning it had to be triggered within several blocks of the bombs, which had been placed in bags and left by the marathon finish line.
They also recovered a 9 mm handgun believed to have been used by Tamerlan from the site of a Thursday night gun battle that injured a Massachusetts Bay Transportation Authority officer.
US investigators had travelled to the predominantly Muslim provence of Dagestan in Russia to speak to the Tamerlan and Dzhokhar's parents Anzor Tsarnaev and Zubeidat Tsarnaeva.
Anzor Tsarnaev has told the Russian state news agency RIA Novosti that the family wants to take Tamerlan's body back to Russia.
Mr Tsarnaev, who described his youngest son as a "true angel", has said that his sons were framed and that they would not have been involved in the attacks.
He and his wife plan to travel to the US.
Investigators are looking into whether Tamerlan Tsarnaev, who spent six months in Russia's turbulent Caucasus region in 2012, was influenced by the religious extremists who have waged an insurgency against Russian forces in the area for years.
The brothers have roots in Dagestan and neighbouring Chechnya but had lived in the US for about a decade.