A line of mourners stretched for half a mile at the Massachusetts Institute of Technology on Wednesday ahead of the memorial service for slain campus cop Sean Collier. Authorities say Collier was shot to death last Thursday by Boston bombing suspects Tamerlan and Dzhokhar Tsarnaev (Tamerlan also died in the shootout). Thousands of police officers, students, MIT faculty, and staff, gathered to pay their respects to Collier, snaking through metal detectors and bomb-sniffing dogs. Along with Collier’s family, Vice President Joe Biden is scheduled to speak at the service. Meanwhile, Boylston Street, the site of the bombings, was reopened to the public Wednesday morning.