Authorities have “mounting evidence” based on “forensic hits” that the two brothers believed responsible for the Boston Marathon bombing were possibly involved in a gruesome, unsolved 2011 triple homicide, ABC News reported on Friday.

According to the report, which cites anonymous law enforcement officials close to the investigation, experts have matched some forensic evidence from the crime scene back to the brothers. Cellphone records also indicate the brothers were in the area when the murders took place.

Tamerlan and Dzhokhar Tsarnaev are believed to have carried out the Boston Marathon bombing that killed three and injured hundreds. Tamerlan, 26, died after a shootout with police, while Dzhokhar, 19, was captured after a citywide manhunt and awaits trial.

Tamerlan’s closest friend, Brendan Mess, was one of three men to die in the 2011 murders. The victims suffered knife wounds to the throat and had marijuana spread over their dead bodies. Buzzfeed reported last month that Tamerlan surprised his friends at the time by not attending the funeral.