The federal government had added the name of the dead Boston Marathon bombing suspect to a terrorist database 18 months before the deadly explosions, US officials are reported to have said.
Five days after the US determined who was allegedly behind the deadly Boston marathon terror attacks, Washington is piecing together what happened and whether there were any unconnected dots buried in the US government files that, if connected, could have prevented the bombings.
"Days after the Boston Marathon bombing suspects were caught and it's now clear that both the FBI and the CIA were aware of Tamerlan Tsarnaev for more than a year," said Al Jazeera reporter Dominic Kane.
"When the hunt for the Tsarnaev brothers concluded last week, police and law enforcement agencies spoke of it as a victory.
"But now the revelation that Tamerlan Tsarnaev was on a watchlist of potential terror suspects might put that in a different light."
The surviving suspect, Dzhokhar Tsarnaev, 19, told authorities that his older brother, Tamerlan Tsarnaev, 26, only recently recruited him to be part of the attack, two US officials said on Wednesday.
Dzhokhar told the FBI that they were angry about the US 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 CIA named Tamerlan to a huge, classified database of known and suspected terrorists a year and a half ago, officials said, an acknowledgment that will undoubtedly prompt congressional inquiry about whether the Obama administration adequately investigated tips from Russia that Tsarnaev had posed a security threat.
Shortly after the bombings, US officials said the intelligence community had no information about threats to the marathon before the April 15 explosions.
The US officials who spoke to The Associated Press were close to the investigation, but insisted on anonymity because they were not authorised to discuss the case with reporters.
Investigators have said the brothers appeared to have been radicalised through materials on the internet and have found no evidence tying them to any one group.
Tamerlan, whom authorities have described as the driving force behind the plot, was killed in a shootout with police.
Dzhokhar is recovering in a hospital from injuries sustained during a getaway attempt.
The CIA made the request to add Tamerlan's name to the terrorist database after the Russian government contacted the agency with concerns that he had become a follower of extremism.
About six months earlier, the FBI had separately investigated Tsarnaev, also at Russia's request, but the FBI found no ties to terrorism, officials said.
Officials say they never found the type of derogatory information on Tsarnaev that would have elevated his profile among counterterrorism investigators and placed him on the terror watch list.
Lawmakers who were briefed by the FBI said they had more questions than answers about the investigation of Tsarnaev.
US officials were expected to brief the Senate on the investigation on Thursday.
Officials said on 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.
Officials said physical evidence, including a 9 mm handgun and pieces of a remote-control device commonly used in toys, was recovered from the scene.
Authorities had previously said Dzhokhar exchanged gunfire with them for more than an hour on Friday night before they captured him inside a boat covered by a tarp in a suburban Boston neighbourhood backyard.
But two US officials said on Wednesday that he was unarmed when captured, raising questions about the gunfire and how he was injured.