This undated file photo released by the Middlesex District Attorney's Office shows Massachusetts Institute of Technology Police Officer Sean Collier, of Somerville, Mass. Investigators said Collier was shot to death Thursday, April 18, 2013, on the school's campus in Cambridge, Mass., by Boston Marathon bombing suspects Tamerlan and Dzhokhar Tsarnaev in a botched attempt to obtain his gun several days after the twin explosions. (AP Photo/Middlesex District Attorney's Office, File)
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