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Boston Marathon Bombing: Tsarnaev Gun Mystery Solved, Say Officials

Officials say the gun used to shoot and kill MIT officer Sean Collier has been traced to a defendant in a federal drugs and firearms case.

Law enforcement officials say they've answered a mystery that's been unsolved since last year's Boston Marathon bombings -- where did Tamerlan and Dzhokhar Tsarnaev get the gun that prosecutors say they used to kill a campus police officer?

Federal prosecutors have filed charges against a Cambridge, Massachusetts man who law enforcement officials say was the source of the gun that was used to murder Officer Sean Collier at the Massachusetts Institute of Technology in Cambridge on April 18, 2013, three days after the Boston Marathon bombing.

Drug and firearms charges were filed against Stephen Silva last week and made public Tuesday. While a federal indictment does not mention the Collier shooting or the Tsarnaev brothers, local and federal law enforcement officials say Silva was the source of the gun used in the shooting.

The indictment says Silva had a Ruger model 95 9mm pistol with the serial number obliterated as of February 2013. That same kind of gun was used to kill Collier two months later, federal officials have previously said. It was also known previously that the gun used to kill him had the serial number obliterated.

The charges unsealed Tuesday also accuse Silva of distributing heroin. Investigators have maintained that Tamerlan Tsarnaev, who died on April 19, 2013, after a shootout with police, was involved in dealing drugs.

-- Pete Williams and Tom Winter