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A makeshift memorial to MIT Police Officer Sean Collier was erected by a steady stream of visitors in front of Building 32 on Friday, April 19, 2013.

Hackers placed the MIT Police badge on both the shoulder and chest of The Alchemist on April 22, 2013 in memory of Sean Collier.

At 2:45 p.m. on Monday, April 22, members of the MIT community stood hand in hand forming a human chain along Vassar Street from the Collier memorial at Stata to the MIT Campus Police station for a statewide moment of...

Hundreds of paper cranes and origami flowers were draped on the MIT police cruiser in the Stata Center in honor of Officer Sean Collier.

A sign for Officer Collier adorns the window of the MIT Police Headquarters on Vassar Street.

A steady stream of visitors paid tribute to MIT police officer Sean Collier in front of Building 32 on Saturday April 20. Collier was slain near this location on April 18 in his police cruiser.

Police officers return to the Cambridge Police headquarters in East Cambridge after Friday’s manhunt was over.

MIT police officer Kevin O’Connor paid tribute to fellow officer Sean Collier in front of Building 32 on Friday, April 19, 2013. Collier was killed near this location the previous night by the Marathon bombing suspects.

The Green Building was lit up in purple on Friday, Apr. 19.

Messages and gifts of thanks and in remembrance of slain MIT police officer Sean A. Collier were displayed at the lobby of MIT Police headquarters.

Messages and gifts of thanks and in remembrance of slain MIT police officer Sean A. Collier were displayed at the lobby of MIT Police headquarters.

Messages and gifts of thanks and in remembrance of slain MIT police officer Sean A. Collier were displayed at the lobby of MIT Police headquarters.

The Green Building at MIT was lit in tribute to slain MIT police officer Sean A. Collier on the night of Apr. 19.

A makeshift memorial to MIT police officer Sean A. Collier was erected by a steady stream of visitors in front of Building 32 on Friday, Apr. 19.

A steady stream of visitors paid tribute to MIT police officer Sean A. Collier in front of Building 32 on Friday, Apr. 19. Collier was slain near this location on Apr. 18.

A steady stream of visitors paid tribute to MIT police officer Sean A. Collier in front of Building 32 on Friday, Apr. 19. Collier was slain near this location on Apr. 18.