Investigators went to 27-year-old Ibragim Todashev's apartment to interview him once again about his relationship with Tamerlan Tsarnaev and his ties to a 2011 triple homicide.
During the interview, authorities say Todashev went crazy and pulled a knife, stabbing an FBI agent several times before officials shot him.
According to ABC News, Todashev was about to sign a statement admitting a role in an unsolved triple murder in Massachusetts on Sept. 11, 2011, one that Boston bombing suspect Tamerlan Tsarnaev is also a suspect in. Three men were found with their throats slit, and cash and marijuana were left on top of their bodies.
Detectives developed DNA evidence linking both Tamerlan Tsarnaev and his younger brother, Dzhokhar, to the murder scene in the wake of the Boston bombings.
Khasuen Taramov, a friend of the man killed, said Todashev and Tamerlan Tsarnaev were acquaintances through mixed martial arts in Boston.
"The older brother, he knew him back like two years ago, back in when he used to live in Boston," Taramov said. "He wasn't like real close friends with him. He just happened to know him, and I guess that was his fault, mistake."
FBI agents and Massachusetts state police began to question Todashev after his name and number were recovered from Tamerlan Tsarnaev's phone. After questioning, investigators found connections to the gruesome murder, but do not believe Todashev was tied to the bombings.
ABC News has learned Todashev was a permanent resident in the U.S. with a Russian passport.
The FBI agent who was stabbed is expected to be fine; the injuries were not life-threatening. An FBI team was dispatched from Washington to review the shooting, a standard step in such cases.
ABC News and The Associated Press contributed to this report.