The congressional delegation sought answers to why Russian warnings about Tsarnaev failed to capture the attention of U.S. authorities.

“It’s tough, I don’t know if they dropped the ball or we dropped the ball, but hopefully we’ll work together better going forward,” Cohen said.

Tamerlan Tsarnaev and his younger brother Dzhokhar are believed to have detonated shrapnel-packed pressure cookers at the finish line of the Boston Marathon earlier this year. The attacks killed three and wounded more than 200 others.

Tamerlan was killed in a gun fight with police. Dzohkhar was captured after a city-wide shutdown and door-to-door search in the Boston metropolitan area. 

Tamerlan spent six months in Russia in 2012, and lawmakers and investigators have sought details about how he became radicalized. Cohen said it was clear from materials the FBI has provided that he was radicalized in the U.S.

Officials in Moscow flagged Tamerlan as a potential threat before his trip to Dagestan, a volatile region in Russia that’s plagued by Islamic violence. The FBI says it requested additional information from Russian officials at the time but didn’t hear back.

U.S. customs agents alerted a joint terrorist task force in the Boston-area upon his return. However, the FBI didn’t follow up on the lead because it had already interviewed him before the trip and closed the case after determining he was not a threat.

Russian officials told the congressional delegation they requested the U.S. notify them if Tamerlan planned to travel to Russia, and that if the U.S. had complied, that they might have been able to take him out before he returned to the U.S. to carry out the bombing.

“[Russian officials] asked for notice about when this guy was coming back to Russia, so I don’t think they knew he was back in Dagestan,” Cohen said. “It’s a surveillance-oriented society, but they didn’t know he was there. If they’d known, maybe they’d have knocked him off. Due process isn't much of an issue for them."