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Three years after the Boston Marathon bombings, there are still a hundred untold stories out there. This is one of them.

After Tamerlan Tsarnaev was killed and his brother, Dzhokhar, was under arrest, the FBI agents and other members of the Marathon task force sat down and drew up a list of gaps in the case. At the top of that list was finding where the gun used to murder MIT police officer Sean Collier came from.

The ATF had been able to raise the obliterated serial numbers on the gun, a Ruger pistol that was recovered at the shootout in Watertown where Tamerlan was killed. But the trail on that gun ran cold in Maine, where someone who once possessed the gun refused to talk.

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In Cambridge, Dzhokhar was known as Jahar, and FBI Special Agent Tim Brown spent months talking to people who knew him. Brown went to Cambridge Police with a theory: Jahar had acquired the gun, and it had something to do with the city’s drug trade. Cambridge PD was all in.

Brown worked mostly antiterrorism, so he turned to Jeff Wood, an FBI agent who was on the bank robbery task force and gang unit, for help. Jahar was well-known for selling marijuana, so they started looking at like-minded dealers who were friendly with him. One kid’s name kept coming up: Stephen Silva.

Silva had been friends with Jahar since middle school. Silva was known to sell marijuana to college students at various local universities. But he was cautious.

The investigation to find the source of the gun began in earnest in November of 2013, seven months after the bombings, but it wasn’t until May of 2014 that it started to bear real fruit. An informant Wood had developed made contact with Silva.

The informant, who has not been publicly identified, was considered extremely valuable.

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“He’s helped us stop homicides, including one of a cop,” Wood said. “He knows what we do. The goal is to stop violence, solve homicides, prevent homicides.”

But Wood didn’t tell the informant why they wanted Silva. Typically, undercover operations like this would pull in three to eight investigators. This one had about 20, including the entire Cambridge PD drug unit.

“You must want this guy bad,” the informant told Wood.

The informant kept meeting with Silva. The informant’s car was wired up, and Wood, parked 75 yards away, listened in on the conversations.

On June 4, 2014, the informant was engaged in small talk with Silva, riffing on celebrities who get in trouble. He mentioned Aaron Hernandez, O.J. Simpson, Martha Stewart.

Down the street, Wood was listening in, wondering, “Martha Stewart? She didn’t kill anyone.”

But the banter worked. Suddenly, out of the blue, Silva told the informant he had given Jahar the gun that was used to murder Sean Collier.

Later, when they sat down for a debriefing, the informant looked at Wood and said, “Now I know why you wanted me to do this.”

They left Silva out there for more than a month, letting him talk more. In the meantime, the FBI agents had an idea.

“We really felt that MIT had to be there when we made the arrest,” Brown said.

They reached out to MIT Police Chief John DiFava, explaining they were hoping that an officer who was close to Collier could assist in the arrest. DiFava told them about Bob Wilcox.

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Wilcox has been on the MIT force for 27 years. He was Sean Collier’s training officer.

It was dark, and Stephen Silva was walking down the street when Bob Wilcox jumped out of a minivan holding handcuffs. Wilcox slapped the cuffs on and told Silva he was under arrest.

Tim Brown walked over to the handcuffed Silva and pointed toward Wilcox, who was wearing his MIT Police rain jacket.

“You see that officer over there?” Brown asked Silva. “Take a look at that officer. Take a look at his jacket.”

With that, Tim Brown walked away.

When Wilcox got back to the MIT Police station, his colleagues were both exuberant and angry: happy to hear they got the guy who supplied the gun, furious that the FBI had shut them out of the case.

Wilcox told his colleagues what really happened.

“At first, they were upset at me for not telling them, but once they realized why, they understood why it was done that way,” Wilcox said.

Wilcox said beyond the symbolism is a greater willingness in the FBI to cooperate and interact with local police departments. It’s about more than respect. It’s about the realities of a world where the next potential attack could be thwarted by a local cop just doing his job.

Silva told his interrogators, and later testified, that he loaned the gun to Jahar because Jahar said he needed it to rob some kids at the University of Rhode Island. Silva’s testimony undercut the defense argument that Jahar was merely following his older, radicalized brother.

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The informant who gave up Silva was paid $66,000 for his work on that case and others. Some of that money was used to relocate him.

Silva was sentenced to time served last December. He went free after 17 months in custody.

Meanwhile, Joe Rogers and his family got some justice.

“Silva was a very important witness,” said Rogers, Sean Collier’s stepfather. “Jahar’s lawyers claimed he was dumb and innocent. This showed he was into this as much as the older brother.”

When Jahar was arrested, the privilege of handcuffing him was given to Transit Police officers, a tribute to their colleague, Dic Donohue, who was shot and nearly died during the firefight in Watertown where Tamerlan Tsarnaev died. Donohue and Sean Collier were friends and had been in the same police academy class.

Rogers said having Wilcox slap the cuffs on Silva was the right thing to do.

“Sean would have approved,” he said.


Kevin Cullen is a Globe columnist. He can be reached at cullen@globe.com. Follow him on Twitter @GlobeCullen.