The Federal Bureau of Investigation on Tuesday will brief the Senate Intelligence Committee on what it learned from an interview it conducted in 2011 with Boston Marathon bombing suspect Tamerlan Tsarnaev, and why it did not pursue him further at the time.

The briefing on Tuesday afternoon will be classified and the witnesses are not yet known, according to a source close to the proceedings.

Tsarnaev, 26, was killed in a firefight early Friday morning after the police tracked down him and his younger brother, Dzhokhar Tsarnaev, 19. They are the only suspects in last week’s Boston Marathon bombing.

A handful of lawmakers in the House, including Speaker John Boehne (R-Ohio) have vowed to conduct briefings and hold hearings into how the brothers operated under the radar of the FBI after the Russian government notified federal investigators that the older brother was a potential threat.

The tip from Russian authorities came after the suspect spent six months in two Russian provinces in 2010 and 2011 before returning to the U.S.