Investigators are pursuing other "persons of interest" possibly linked to the fatal Boston Marathon bombings, US lawmakers said.
"There are still persons of interest in the United States that the FBI would like to have conversations with," House Intelligence Committee Chairman Mike Rogers told ABC's This Week. The Michigan Republican declined to indicate how many people were being sought.
Dutch Ruppersberger, the top Democrat on the panel, said investigators were scouring telephone logs that took place before and after the April 15 twin blasts that killed three people and wounded more than 260.
Authorities have identified two brothers - Tamerlan and Dzhokhar Tsarnaev - as suspects.
Tamerlan died during a shootout with police days after the attacks, but his 19-year-old brother was captured alive and is being held at a federal prison medical centre outside Boston.
Representative Michael McCaul, head of the House Homeland Security Committee, said the suspects probably received training from extremists, considering the "level of sophistication" of the device used.
Investigators believe the Tsarnaevs' pressure-cooker bombs were probably detonated by long-range remote controls.
McCaul also stressed that pressure-cooker bombs were a "signature device" used in Pakistan and Afghanistan.
"The way they handled these devices and the trade craft leads me to believe that there was a trainer," he said on Fox News Sunday. "And the question is, where is that trainer or trainers? Are they overseas in the Chechen region or … in the United States?"
The lawmakers' comments came as US media reported that Russian authorities secretly wiretapped the mother of the brothers, Zubeidat Tsarnaeva, and recorded her discussing jihad in vague terms during a 2011 telephone conversation with Tamerlan. Even though the conversation took place two years ago, the Russians only turned over the information to their US counterparts in recent days, according to the reports.
The investigation has also focused on a mysterious man named Misha who is said to have influenced Tamerlan and possibly encouraged him to go down a radical path.
Both the CIA and the FBI flagged Tamerlan over possible ties to terrorism after Russian officials contacted the US agencies in 2011.
Reports said Russian authorities had also alerted US counterparts about concerns that his mother was a religious extremist, and that she was added along with her older son to a "terror watchlist".
Rogers said Moscow possessed other information that would be "incredibly helpful."