BOSTON - It was a shocking slaying in a Boston suburb that sometimes goes years without one homicide, let alone three at once. The victims' throats had been sliced in a home on a tree-lined street, marijuana and cash strewn over their bodies.

The investigation languished for more than 18 months until just a few days after the Boston Marathon explosions, when the FBI identified the suspected bombers as Tamerlan Tsarnaev, 26, and his brother, Dzhokhar Tsarnaev, 19, ethnic Chechens who emigrated from Russia to the U.S. about a decade ago.

The 2011 homicide case in Waltham remains unsolved, though Tamerlan Tsarnaev was implicated in the killings after his death in a post-bombing shootout with police. Clues in the Waltham killing that might early on have led local investigators to Tamerlan Tsarnaev, who was already on the FBI's radar as a possible religious extremist, raise questions about whether the marathon bombings could have been averted.

"Let's put it this way: If they had arrested Tamerlan as one of the killers in the triple killing before the marathon bombing, it certainly would have affected the outcome of the marathon bombing," said Waltham City Councilman Gary Marchese. "He would have been in jail and would have been tried for murder."

Tamerlan Tsarnaev and one of the slaying victims, Brendan Mess, were both boxers and sparred often at a local gym. Despite what friends described as a close friendship, Tamerlan did not attend Mess' funeral. And the owner of a restaurant where Mess was a regular and brought Tamerlan and another victim along with him a few times says police never interviewed him.

The brothers lived in Cambridge, across the Charles River from Boston. In March 2011, months before the September killings to the west in Waltham, Russian authorities had told the FBI they worried that Tamerlan Tsarnaev and his mother, who lives in Russia, were religious extremists. The Russians were unresponsive when pressed for more details. The FBI didn't find any derogatory information on Tsarnaev, and a criminal case was not opened. The FBI shared its results with Russia in summer 2011, shortly before the Waltham killings.