WALTHAM, Mass. – A year and a half before the Boston Marathon bombing, the man Tamerlan Tsarnaev called his best friend died in a grisly crime: He was one of three men found nearly decapitated in an apartment, their throats slashed ear to ear, marijuana sprinkled over their bodies.

Now, in an offshoot of the marathon bombing investigation, authorities are trying to determine whether the now-dead Tsarnaev had a hand in the unsolved triple slaying in this Boston suburb.

The direction of the investigation came into sharper focus on Wednesday, when another friend of Tsarnaev’s, Ibragim Todashev, was shot to death in Orlando, Fla., after allegedly trying to attack an FBI agent while he was being interrogated.

Two officials, speaking on condition of anonymity because they were not authorized to release details of the investigation, told The Associated Press that Todashev had implicated himself in the 2011 Waltham killings before he was shot.

The officials said they did not know if Todashev had also implicated Tsarnaev.

After the April 15 bombing at the Boston Marathon killed three people and wounded more than 260, Massachusetts prosecutors said they would investigate any links between Tsarnaev and the Waltham killings.

Tsarnaev died April 19 after a shootout with police. His younger brother, Dzhokhar, was captured and charged in the bombing.

Prosecutors would not discuss the triple slaying or whether they believe Tsarnaev and Todashev were involved.

Middlesex District Attorney Marian Ryan said Thursday that her office is working closely with state police, Waltham police and the FBI.

“While we cannot discuss details pertaining to the investigation, including evidence, suspects or witnesses, this office and its law enforcement partners have conducted a thorough, far-reaching investigation beginning in 2011 when this horrific crime occurred. This investigation has not concluded and is by no means closed,” Ryan said in a statement.

One of the victims, 25-year-old Brendan Mess, was a boxer, like Tsarnaev. Tsarnaev described Mess as his “best friend” to the owner of the gym where he and Mess worked out, The Boston Globe reported.

The motive for the killings is unclear. At the time, prosecutors said they believed the victims knew their killer or killers. The killers left behind thousands of dollars in Mess’ Waltham apartment, where the bodies were found.

Christopher Medeiros, who described himself a close friend of Mess, said he believes the killings were drug-related. He said Mess and another one of the victims, Erik Weissman, were marijuana dealers and had been trying to start a major growing operation.

“The Friday before he died, (Mess) told me, ‘Listen, I’m getting ready to make this big move,”‘ Medeiros said. “And I think that’s what cost him his life.”

Medeiros said he did not see any signs of tension between Tsarnaev and Mess.

Medeiros said Mess began spending time with Tsarnaev in late 2010 or early 2011.