Ibragim Todashev, a friend of deceased Marathon bombings suspect Tamerlan Tsarnaev, told investigators that Tsarnaev participated in a triple murder in Waltham in 2011, according to a filing by prosecutors in federal court.

Todashev was killed by an FBI agent who was interviewing him in May. His death remains under investigation. Tsarnaev was killed when he was shot by police and run over by his own brother in a confrontation in Watertown several days after the April 15 bombings.

After the bombings, investigators began taking a closer look at whether Tsarnaev, who knew the Waltham murder victims, was involved in that case.


Todashev’s statement that Tsarnaev was involved in the murders was previously reported. But the court filing was the first official confirmation.

“According to Todashev, Tamerlan Tsarnaev participated in the Waltham homicides,’’ prosecutors said in the filing, though they did not elaborate.

The federal prosecutors’ statement came in response to a motion filed by defense attorneys for Dzhokhar Tsarnaev that sought to compel the release of prosecution evidence. Dzhokhar Tsarnaev, Tamerlan’s younger brother, faces federal charges and sought the evidence to prepare his defense, but prosecutors opposed the motion, which included, among other things, a request for more information about the Waltham case.

“The Middlesex District Attorney’s office is engaged in an active, ongoing investigation into the Waltham triple homicide,’’ prosecutors said. “Disclosure of the details of that investigation could jeopardize it.’’

“Any benefit to Tsarnaev of knowing more about the precise ‘nature and extent’ of his brother’s involvement does not outweigh the potential harm of exposing details of an ongoing investigation into an extremely serious crime, especially at this stage of the proceedings,’’ prosecutors said.

The three Waltham victims’ throats were slit in the early evening hours of Sept. 11, 2011, the Globe reported in June.

For the next 18 months the investigation seemed to go nowhere. The killings were dismissed by many as what appeared to be three low-level drug dealers who must have gotten in over their heads.


Then, after the bombings, investigators began taking a closer look at Tamerlan Tsarnaev, who, along with his brother, is accused of killing three people and wounding 260 others in the twin blasts that struck near the Marathon finish line, as well as the murder of an MIT police officer.