Chechen was killed by FBI agent while being interviewed about ties to one of the Boston Marathon bombing accused

State investigators in Florida have rejected a request for an independent investigation of the fatal shooting of a Chechen man while he was being questioned about his ties to one of the Boston Marathon bombing suspects.

Florida's department of law enforcement declined the request by the American Civil Liberties Union of Florida to look into the death of Ibragim Todashev.

Todashev was killed in May while being questioned by FBI agents and others. Officials originally said Todashev lunged at an FBI agent with a knife. They later said it was no longer clear what happened.

Tuesday's letter said it would be inappropriate for the Florida agency to intervene.

ACLU of Florida director Howard Simon said he found the response extremely disappointing. "Secrecy fosters suspicion and the people of Florida deserve better than to be left without an explanation from their government about what led to a person being shot to death," Simon said in a news release.

"This means it is more likely that the family of Ibragim Todashev will have to file a lawsuit in order to get answers about how their son died."

Dzokhar Tsarnaev, 19, has been charged over the Boston Marathon bombings of 15 April in which three people died and more than 260 were injured. He was captured by police four days later after a gunfight and chase in which the other suspect, his brother, Tamerlan Tsarnaev, was killed.

The FBI had been questioning Todashev about links to Tamerlan Tsarnaev and their possible involvement in a triple murder in the Waltham area west of Boston when he was killed by an agent. Authorities have said the agent retaliated after being attacked by Todashev who had just confessed to the murders – though the details given by officials have varied.