An FBI agent has been cleared over the fatal shooting of a Chechen-born man being quizzed in connection with last year's Boston bombing, a law enforcement source said Friday.

Mixed martial artist Ibragim Todashev, 27, was shot dead after a violent altercation with investigators following two hours of questioning at his apartment in Orlando, Florida in May last year.

Police later said Todashev, a friend of slain Boston bomber Tamerlan Tsarnaev, had pulled a knife on investigators and tried to grab the gun of an FBI agent following questioning.

Todashev had confessed to his and Tsarnaev's involvement in a September 11, 2011 triple murder before he was shot, authorities said.

Todashev's father had challenged the official version of events, questioning how his son could have obtained a knife when there were as many as six police officers in his apartment at the time.

But a law enforcement source familiar with the shooting said an investigation had exonerated the FBI agent involved.

"The State and the FBI's review cleared the shooter of any wrongdoing," the source said.

The Department of Justice, which is also investigating the death, is expected to follow the FBI's recommendation, the source said.

Todashev had been questioned several times in the aftermath of the April 15 bomb attack on the Boston Marathon carried out by Tsarnaev and his brother which left three people dead and 264 injured.

Todashev was a known associate of Tsarnaev because the two men trained at the same mixed martial arts gym in Boston, a friend, Khusn Taramiv, told local television.

Tsarnaev, 26, was killed three days after the attack following a gun battle with police. His brother, Dzhokhar Tsarnaev, 19, was arrested following a vast manhunt several hours later.

He faces the death penalty if convicted at his trial, which opens in November.

[Image via Agence France-Presse]