The Boston marathon bombing might never have happened if the police had asked a murder victim’s girlfriend one obvious question back in 2011.

The victim was 25-year-old Brendan Mess, who had been found with his throat slashed, along with two other men murdered in a similar fashion, in a Waltham, Mass., apartment.

His girlfriend, who prefers to be unnamed, was one of at least five people who told the police that Tamerlan Tsarnaev was a close friend of Mess. She reportedly told police that Tsarnaev was a regular visitor to the apartment.

That should have been the cue for the police to ask the question that could have changed everything.

“What else can you tell us about Mr. Tsarnaev?”

The girlfriend then might very well have told them something that she has since told the Boston Globe, something that would have given even the dimmest cops reason to consider Tsarnaev a suspect, given that the killings took place on the 10th anniversary of the 9/11 attacks.

Shortly before the killings, the girlfriend says, Mess confided to her that Tamerlan had been interrogated by the FBI. Tsarnaev had told Mess said the agents seemed to suspect him of being a terrorist and supposedly had placed him on some kind of watch list.

The girlfriend, who is Muslim, recalls that she and Mess had just laughed. The idea of Tsarnaev being a terrorist had seemed ludicrous, even though he did sometimes chide her for drinking and failing to cover her head and otherwise not acting as he felt a woman of his faith should.

Had the police taken a step back, they might have considered the possibility that Tsarnaev had come to view his Christian friend, Mess, as a corrupter. The other two victims of this September 11 massacre were Jewish.

The police would have only needed to call the FBI to confirm that Tsarnaev had indeed been questioned, at the behest of Russian intelligence. The FBI had concluded that there was nothing to warrant further investigation, but the agents might well have reconsidered had they learned of the triple murder and its timing.

One thing the girlfriend apparently did tell the police was that there had been considerably more cash in the apartment than the $5,000 that was actually vouchered. The money had apparently come from selling marijuana, a quantity of which had been scattered over the bodies.

The girlfriend also told the police that Mess had acquired a handgun after previously being beaten by a marijuana supplier. That gun, possibly the very one that Tsarnaev would later use to kill one police officer and seriously wound another, was missing from the apartment.

Instead, the police deemed the murders a drug ripoff that involved people who knew each other, noting that there were no signs of forced entry. The police also said that two unidentified people had been seen in the vicinity around the time of the killings.

But the police apparently failed to take even cursory steps to investigate whether Tsarnaev might be one of those people. The police would have been taking a step toward identifying both of the people in question if they had bothered to visit the Wai Kru mixed martial arts gym where Mess worked out.

Wai Kru’s owner, John Allan, could have told police that Mess and Tsarnaev had once considered each other best friends. Allan also could have mentioned that Tsarnaev had shown a surprising lack of grief after Mess was murdered, instead half smiling and saying “that’s what can happen.” Tsarnaev had attended neither the wake nor the funeral of his onetime best friend and started telling people he had no American friends at all.

The owner could have informed the police that Tsarnaev was a gym buddy with Ibragim Todashev, who was given to flashes of temper and anti-American outbursts. The two would work out together and then bow toward Mecca to pray as Tsarnaev became increasingly fundamentalist in his beliefs.

After the Boston bombing and the wild shootout and manhunt that resulted in Tsarnaev’s death on April 19, FBI agents learned the 26-year-old frequented the Wai Kru gym and they paid the place the kind of visit the police should have made after the triple murder two years before. The owner told the FBI what he would have told the cops, including about Tamerlan’s friendships with Mess and with Todashev.

The FBI noted numerous cellphone and email communications between Tsarnaev and Todashev, who had since moved to Florida. Todashav was repeatedly questioned at an FBI office there regarding any possible involvement in the bombing, which he steadfastly denied.

Todashev finally told the agents that he was fed up and that they would have to come to him if they wanted to ask him anything else. The FBI and Massachusetts authorities had by then looked into the link with Mess enough to suspect what the police should have come to suspect in 2011. Two Massachusetts state troopers joined the two FBI agents who went to went to see Todashev at an Orlando co-op on May 22.

Todashev sat at the kitchen table while an agent across from him questioned him for hours. Todashev continued to deny any involvement in the bombing, but he did admit to joining Tsarnaev in committing the triple murders, senior law-enforcement officials say.

The agent across from Todashev is said to have presented him with a pad and he began to write a confession. The state trooper standing behind and slightly to the right of Todashev noted the 27-year-old Chechen immigrant was becoming increasingly agitated. And Todashev’s recent arrest record had given some insight into his volatility. He had gone into a blind rage over a parking space and given the object of his fury a terrible beating.

According to these law enforcement officials, the trooper did not want to interrupt the momentum of the confession by calling for a break. He decided instead to text a warning to the agent that the guy seemed ready to blow. As the agent looked down to read the message, Todashev apparently took the unguarded instant as opportunity. Todashev leapt up and pushed back the table with such force that the agent cracked his head against the wall.

The stunned agent would later say that he had his gun in his hand before he fully realized he had drawn it. Todashev reportedly came at him with what looked like a metal pipe, but reportedly proved to be an aluminum broom handle. The agent fired twice. Todashev kept coming. The agent fired as many as four more times and Todashev fell mortally wounded.

The written confession was left forever unfinished, but Todashev had given a full verbal account. The FBI has not revealed what Todashev said was the exact motive for the murders, though it seems that for Tsarnaev it was a way to cast off the last vestige of his westernized, party-hearty ways by killing his American pothead friend and two Jews, jihadi style, by slashing their throats on the 10th anniversary of 9/11.

If the killings were indeed also a robbery, the FBI and the state police have no doubt compared the amount that Todashev said he and Tsarnaev took with what Mess’s girlfriend estimated was in the apartment.

Otherwise, the internal affairs bureau of the Waltham police is no doubt putting in extra hours in an effort to determine what happened to the difference. The Waltham police referred questions about the case to the Middlesex district attorney’s office, which offered no comment regarding an “ongoing investigation,” as it has been called since it was going nowhere at all. The police certainly need no more scandal—their chief was forced to resign this week after being convicted of battering his wife.

Whatever happened to whatever money was there, the new chief should investigate why his investigators showed so little enterprise in a triple murder. At least five people say they told the police about Mess’s relationship with Tsarnaev. Yet the police do not seem to have made the slightest effort to follow that lead.

A conspiracy-minded person might suggest the police did not want the case solved. But that becomes all the more unlikely when you consider that the investigation was overseen by Gary Leone, then the Middlesex County district attorney. He also happened to be a senior member of the joint terrorism task force. He resigned just before the bombing to become a partner in the same law firm where former U.S. Sen. Scott Brown landed.

Were he still in office, Leone might be facing tough questions about whether a more aggressive investigation into the triple murders might have prevented the Boston bombing, as well as the killing of a police officer. Who knows, if the police had gathered enough against Tsarnaev to secure a search warrant in 2011, they might have recovered a gun before it harmed anybody.

Perhaps the police had tunnel vision, fixated from the beginning on the belief that the killers were drug dealers. And maybe the police were just plain lazy, telling the mother of one victim that it was just a question of time before one of the killers got arrested for something and gave up the others. Drug-related killings, like those involving prostitutes, do not always rouse police to their most strenuous and impassioned efforts.

In this case, the investigators seem not to have asked that simple question that might have changed everything.