Waltham, Massachusetts (CNN) -- Jamal Abu Rubieh, owner of the Brookline Lunch cafe, knew something wasn't right when Brendan Mess and Erik Weissman sat quietly in a back corner booth that August night in 2011.
Gone was the usual boisterous banter typical of their regular visits. Abu Rubieh says things became even more tense when a bald, blue-eyed man twice their age arrived. The three talked for about an hour.
"He sounded different and he acted different, the guy with them, and they all were, like, nervous," Abu Rubieh said. "And that time, (Mess) was really serious and he wasn't himself."
The man, who Abu Rubieh had never seen before and hasn't seen since, had a thick Boston accent. Three weeks later, on September 11, Mess, 25; Weissman, 31, and another friend, Raphael Teken, 37, were found dead in their second-floor Waltham apartment. Their throats had been slashed ear-to-ear, marijuana was sprinkled across the corpses, and several thousand dollars in cash was found at the scene.
A neighbor says Mess' girlfriend discovered the bodies and came to his house. Her feet were bloody, and she was crying. She calmly asked for a cigarette and called police.
Then-District Attorney Gerry Leone said days after the crime that the victims and two unknown perpetrators appeared to have known each other and that it was not a random crime. No suspects were named, no arrests were made.
The case got no attention in the national media at the time. But nearly two years later, when Mess' friend Tamerlan Tsarnaev was named as a suspect in the Boston Marathon bombings, authorities began to take a closer look at the unsolved murders.
The victims were fun-loving guys, by their friends' accounts.
Weissman had a business making high-end glass bongs and was planning to move to California once his drug charges in Massachusetts were resolved. His lawyer, Norman Zalkind, says his client was close to a plea deal when he was killed, but declined to say more, citing attorney-client privilege.
Weissman, Teken and Mess were "always together," said Abu Rubieh, but he didn't know how the three came to know each other. Mess' picture still hangs on the restaurant wall. A dish called the Brendan Benedict was named in his honor.
Nearly two years later, many who knew the victims are puzzled by the fact that they weren't questioned by detectives investigating the killings. They're also wondering why police didn't question Tamerlan Tsarnaev.
Mess and Tsarnaev were high school friends and sparring partners at John Allan's Wai Kru gym in the Boston neighborhood of Allston. But Allan said when he talked to Tsarnaev several days after Mess' killing, his friend showed no signs of grief.
"He kind of laughed it off, saying, you know, that Brendan probably got what he deserved ... making bad choices, that those were the repercussions he had to face," Allan said.
Though only Weissman had faced a drug charge, Allan said the assumption was that the murders were connected to drugs.
"Around here, we call it 'NHI' -- no humans involved," Allan said. Meaning,he explained, that "there were three drug dealers murdered over drugs and money, which seems apparent because that was what was left over the bodies."
But Bobby Black, a friend of Weissman and a writer for the pro-marijuana magazine High Times, said the theory of a drug robbery makes little sense when no drugs or money appear to have been taken from the apartment where the men were killed.
"Anyone who knew Erik would know that he was in no way some kind of dangerous drug dealer," Black said. "He was a college-age kid who loved weed. I think that the police writing it off early on possibly may be the reason they didn't investigate further, which could have prevented the Boston bombings."
The killings -- on the 10th anniversary of the 9/11 terror attacks -- occurred as Tsarnaev, who came from a family of Chechen refugees, was becoming an increasingly devout Muslim. Allan said Tsarnaev and another gym member, Ibragim Todashev, used to pray together in the gym.
Allan said he was never questioned by police after the gruesome murders of the three friends. Neither was Abu Rubieh, who says he had a father/son relationship with Mess and who knew Tsarnaev as well.
Four months after the murders, Tsarnaev left Boston and traveled to the Russian republic of Dagestan, where his parents lived. Sources have told CNN they believe Tsarnaev became radicalized on that trip, and some law enforcement sources have questioned whether the marathon bombings would have happened had Tsarnaev been questioned in the Waltham killings.
Authorities accuse Tsarnaev, 26, and his younger brother, Dzhokhar, 19, of planting the bombs that exploded April 15 at the finish line of the Boston Marathon. Three people were killed there and more than 260 wounded. While Tamerlan Tsarnaev was killed a few days later as he and his brother engaged in a firefight with police, Dzhokhar was captured. He was arraigned last week on charges of killing four people -- including an MIT police officer who was shot as the brothers fled -- and wounding hundreds more in the bombings and the pursuit. He pleaded not guilty.
In the ensuing investigation of the marathon bombings, authorities also questioned Todashev, who had moved to Florida.
While being questioned in his home May 22 by an FBI agent and Massachusetts state police officers, Todashev admitted he played a direct role in the Waltham killings and implicated Tamerlan Tsarnaev as well, a federal law enforcement source told CNN at the time.
Todashev told investigators the men were killed during a drug ripoff because he and Tsarnaev were afraid they would be able to identify them and tell police what happened, according to a law enforcement source.
A U.S. government official briefed on the investigation said that while being questioned, Todashev suddenly knocked over a table, brandished "a long-handled object" and lunged at the FBI agent, who shot and killed him. The shooting remains under investigation.
Middlesex County District Attorney Marian Ryan said after Todashev's death that the Waltham killings remain an "open and active" case, and that police and prosecutors "have conducted a thorough, far-reaching investigation."
"This investigation has not concluded and is by no means closed," Ryan said.
Now some of those who knew Weissman, Teken and Mess are questioning whether Tsarnaev should have been under scrutiny long before the marathon attack.
Looking back at the young man's attitude following the Waltham murders, Allan wonders whether a big clue was missed.
"Retrospectively, maybe there was something behind that," he said.
CNN's Susan Candiotti, Carol Cratty, Matt Smith and Stephen Samaniego contributed to this report.