Authorities say there is "mounting evidence" that the Tsarnaev may have been involved in an unsolved triple murder in 2011.
Massachusetts law enforcement told ABC News that there were "forensic hits" that show potential involvement of Tamerlan and Dzhokhar Tsarnaev in the grisly murder of Tamerlan's friend Brendan Mess and two others.
It is too early to say definitively without more DNA testing whether they were involved in the crime, police said.
Cell phone records of the brothers put them near the crime scene that night but few other details were offered about evidence.
Brendan Mess, Erik Weissmann and Raphael Teken were killed on September 11, 2011 in Waltham, Mass., the town next to Watertown.
The victims had ordered food but when it arrived 20 minutes later nobody answered the door.
The bodies were found the next day - their throats slashed, covered in seven pounds of marijuana and $5,000 cash.
Authorities suspected it was drug-related but no leads emerged and the case went cold. Investigators said they believed the killings were personal and that an accomplice was at the scene.
Mess was a friend of Tamerlan Tsarnaev and they worked out at the same gym together. They were even roommates at one point.
Tsarnaev had even described him as his "best friend" but oddly, he did not show up at Mess' funeral nor ever return to the gym where they trained together.
Authorities began exploring the link between Tamerlan Tsarnaev after the Boston bombings.
ABC News said that officials have declined to comment on the record as the investigation is ongoing.