Several friends said in recent interviews that they told the police about Mr. Tsarnaev when they were questioned. “The police wanted to know who all the friends were in the group, and I told them about Tamerlan,” said one close friend of Mr. Mess, adding that at least three other friends gave the authorities Mr. Tsarnaev’s name, as well.

Image Ibragim Todashev, a friend of Mr. Tsarnaev's, was killed in May. Credit... Orange County Sheriff's Office, via Getty Images

When Mr. Tsarnaev did not show up at either Mr. Mess’s funeral or memorial service, the friend became uneasy.

“We did mention Tamerlan again to the police after he was not there for Brendan’s services,” said the friend, who spoke on the condition of anonymity because of the notoriety of the case. “I felt that the police were not really looking in the right places.”

In the immediate aftermath of the murders, investigators theorized that the killings had been the work of professionals, based on the savageness of the attacks on the three victims, at least two of whom were adept at martial arts, and the lack of evidence at the scene. One early theory was that the assailants might have been part of a “cartel” that felt betrayed by one of the men, according to two law enforcement officials. They said that at least two people had been at the apartment near the time of the deaths, that the killers most likely knew their victims, and that the homicides were not random.

Since the bombings, some Jewish publications have speculated that because the bodies were discovered the day after the 10th anniversary of the Sept. 11 terrorist attacks, that they could have been a hate crime, particularly since both Mr. Weissman and Mr. Teken were Jewish. “I told the police that there were only two Muslims I knew of who hung around the group of friends, and because of the 9/11 date, the fact that Tamerlan and Brendan’s last girlfriend were Muslim stood out,” the friend said.

Mr. Weissman’s mother, Bellie Hacker, recalled meeting with investigators about 10 days after the murders, describing them as “honest in their assessments, but passive and waiting.”