The CIA and the FBI were aware of suspected Boston Marathon bomber Tamerlan Tsarnaev at least a year before the attacks but neither followed up initial inquiries, it has emerged.
The latest information gives a glimpse of the complex relationship and differing roles of the agencies.
It also reveals that they were aware of the 26-year-old for at least 12 months before he and his brother Dzhokhar, 19, allegedly planted bombs that killed three people and injured about 260, some critically, near the marathon's finish line on 15 April.
Counter-intelligence officials were first alerted to Tamerlan Tsarnaev more than a year ago. Russian authorities contacted the CIA and FBI over concerns that he may have been planning an attack in Russia, the BBC reports.
The FBI opened a preliminary investigation and asked for further information, but found no evidence of any threat to the United States. In compliance with legal constraints, the file was closed.
Officials said Tamerlan Tsarnaev had been added to the Terrorist Identities Datamart Environment (Tide) at the request of the CIA. The database contains as many as 745,000 entries, and individuals on that list are not necessarily on the so-called terrorist watch list.
US lawmakers have been briefed about the investigation and say all the proper protocols appear to have been followed.
Following the bombing, Tamerlan Tsarnaev was killed in a shootout with police after an campus officer at the Massachusetts Institute of Technology was shot on 18 April.
It sparked a huge manhunt that ended with the capture of Dzhokhar Tsarnaev the following day. He was seriously injured and remains in a Boston hospital.
He is under arrest awaiting trial and has been charged with using a weapon of mass destruction and malicious destruction of property resulting in death. He may face the death penalty if convicted on either count.
Memorial held for MIT officer
A memorial service was held on Wednesday for the university police officer killed during the hunt for the Tsarnaev brothers.
Vice-President Joe Biden was among those who attended to honour Sean Collier, 26, at the Massachusetts Institute of Technology.
About 4000 students, MIT staff and police officials from around the country were present. Queues of mourners stretched for almost 1km before the ceremony started.
Mr Collier's brothers thanked police and others for their support.
Mr Biden told the family: "My heart goes out to you. I hope you find some solace in this time of extreme grief."
Mr Collier was buried on Tuesday.
