Family, friends and police officers gather at the Collier Memorial following a ceremony at the Massachusetts Institute of Technology in Cambridge, Mass., Wednesday. The Associated Press

Campus police officers march past the memorial erected at the site where Officer Sean Collier was fatally shot by the Boston marathon bombers. The Associated Press

Two guests lay roses on the Collier Memorial at MIT in Cambridge, Mass. The Associated Press

MIT has dedicated the permanent memorial in honor of campus police Officer Sean Collier. The Associated Press

This undated file photo released by the Middlesex District Attorney's Office shows Massachusetts Institute of Technology Police Officer Sean Collier, of Somerville, Mass., was shot to death April 18, 2013, on the school's campus by Boston Marathon bombers Tamerlan and Dzhokhar Tsarnaev in a botched attempt to obtain his gun several days after the twin explosions. During the trial of Dzhokhar Tsarnaev, MIT Police Chief John DiFava testified he told Collier to "be safe" about an hour before he was shot dead. the Associated Press