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Associated Press
FILE - This undated file photo provided by the Middlesex District Attorney's Office shows Massachusetts Institute of Technology Police Officer Sean Collier, 26, of Somerville, Mass., who was shot to death Thursday, April 18, 2013 on the school campus in Cambridge, Mass. Stephen Silva, a friend of Boston Marathon bombing suspect Dzhokhar Tsarnaev, was arrested Tuesday, July 22, 2014, and is believed to have provided the handgun used to kill Collier. (AP Photo/Middlesex District Attorney's Office, File)

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