WALTHAM, Mass. — It was the most brazen crime in the memory of this Boston suburb: three men murdered with knife slashes to their throats in a second-floor apartment at 12 Harding Ave.; each corpse precisely positioned, stomach down, head turned a quarter to the right, marijuana sprinkled on top.

The case has remained unsolved since the bodies were discovered on Sept. 12, 2011. It is now back in the investigative spotlight as evidence mounts, according to law enforcement officials, that one of the suspects in the April 15 bombings at the Boston Marathon, Tamerlan Tsarnaev, participated in the killings.

The emerging evidence againstTsarnaev, who died April 19 after a shootout with the police, has led some law enforcement authorities to contend that if the local murder investigation had been more vigorous it could have led to his apprehension well before the bombings left three dead and more than 260 wounded — in short, that the bombings might never have happened.

Even before Ibragim Todashev, his friend and boxing partner, implicated himself and Tsarnaev prior to being killed during an FBI interrogation in late May, federal investigators had been building a case against Tsarnaev using “old-fashioned” police work, a senior law enforcement official said. They “went through his phone, did interviews, got his contacts — a combination of all that.” They believe the two men targeted the victims, one of whom was a close friend, in a drug-related robbery.

Relatives and friends of the Waltham murder victims — Brendan Mess, 25, Erik Weissman, 31, and Raphael Teken, 37 — have long contended that the authorities were too quick to write off the murders as the unfortunate outcome of a low-level drug dispute and that they failed to do basic policing work that could have solved the case.

Mess, a popular martial arts instructor, was believed to have been running a drug-dealing operation from the apartment. Weissman, a chess player and Chinese food lover who was a founder of a glass pipe company and had been temporarily staying at the apartment, was working on a plea deal stemming from an earlier drug arrest. Teken, known as Rafi, was a 1998 graduate of Brandeis University.

Tsarnaev, 26, was, by many accounts, one of Mess’ closest friends and a frequent visitor to the Harding Avenue apartment. But three law enforcement officials familiar with the case said that Massachusetts state troopers and the Waltham police, working under the auspices of the Middlesex County District Attorney’s Office, never questioned him — either as a potential suspect or as someone who could provide valuable information about the victims.

Several friends said in recent interviews that they told the police about 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 Mess, adding that at least three other friends gave the authorities Tsarnaev’s name, as well.

When Tsarnaev did not show up at either Mess’ 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 Weissman and 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.

Weismann’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.”

“The police told us, ‘This is what we think may happen.’ That in the future, someone with information might come forward and admit it and seek a plea deal,” Hacker said.

Gerry Leone, who oversaw the Waltham investigation as the district attorney until leaving his post in mid-April to join a law firm, declined to comment on the inquiry, as did a spokeswoman for the prosecutor’s office and a spokesman for the state police. One law enforcement official involved in the case denied that anyone suggested that they look at Tsarnaev in the aftermath of the killings: “Don’t you think if someone had told us to take a look at him and that he had information, we would have talked to him?”

One person whom investigators did interview on several occasions was Mess’ most recent girlfriend, a Muslim like Tsarnaev. Some friends and relatives of the victims are suspicious of her, and in an interview for this article, she acknowledged that she had a fiery relationship with Mess at times and had once thrown a bottle of balsamic vinegar at him.

The girlfriend, who spoke on the condition of anonymity because she did not want her name associated with the case, insisted that she had no part in the murders and had hoped to marry Mess, whom she started dating about nine months before his death. She said that she believes she told investigators that Tsarnaev, whom she knew as “Tam,” was a regular guest at the Waltham apartment, but that the authorities never followed up with her about him.

She added that parts of her boyfriend’s relationship with Tsarnaev were cloaked in mystery, citing as one example a conversation the two men had shortly before she left on a trip to Miami the week before the killings.

“Tam asked Brendan, ‘Are we going to do that thing?’” she recalled. “And I asked Brendan what that was, and he told me not to worry about it.”

A Massachusetts legislator who talked to neighbors the day the bodies were found and has closely followed the case, Rep. John Lawn, asserted that the investigation was “thoroughly done.”

Since there were no signs of forced entry, “it had to be someone they trusted to let them into that apartment,” he said. But the lack of clues fueled what he called “the consensus” that developed about the murders. “It was very professionally done,” he said.