In this 2010 photo provided by the Massachusetts Bay Transportation Authority, Richard Donohue Jr., left, and Sean Collier pose together for a photo at their graduation from the Municipal Police Officers' Academy. On Thursday, April 18, 2013, Massachusetts Institute of Technology Police Officer Collier was fatally shot on the MIT campus, and transit police officer Donohue was shot and critically wounded. Authorities allege that Boston Marathon bombing suspects Tamerlan and Dzhokhar Tsarnaev were responsible. (AP Photo/Massachusetts Bay Transportation Authority)
In this 2010 photo provided by the Massachusetts Bay Transportation Authority, Richard Donohue Jr., left, and Sean Collier pose together for a photo at their graduation from the Municipal Police Officers' Academy. On Thursday, April 18, 2013, Massachusetts Institute of Technology Police Officer Collier was fatally shot on the MIT campus, and transit police officer Donohue was shot and critically wounded. Authorities allege that Boston Marathon bombing suspects Tamerlan and Dzhokhar Tsarnaev were responsible. (AP Photo/Massachusetts Bay Transportation Authority)
In a photo provided by Massachusetts Institute of Technology, MIT police pallbearers carry the casket of fallen MIT officer Sean Collier during a memorial service on the MIT campus in Cambridge, Mass., Wednesday, April 24, 2013. Collier was fatally shot on campus Thursday night, April 18, 2013. Authorities allege that the Boston Marathon bombing suspects were responsible. (AP Photo/Massachusetts Institute of Technology, Dominick Reuter)
Andrew Collier, left, puts his hand on his brother, Robert, after delivering the eulogy at a memorial service for their brother, slain Massachusetts Institute of Technology campus officer, Sean Collier, at MIT in Cambridge, Mass. Wednesday, April 24, 2013. Sean Collier was fatally shot on the MIT campus Thursday, April 18, 2013. Authorities allege that the Boston Marathon bombing suspects were responsible. (AP Photo/Elise Amendola)
Police officers salute during a memorial service for fallen Massachusetts Institute of Technology campus officer Sean Collier at MIT in Cambridge, Mass. Wednesday, April 24, 2013. Sean Collier was fatally shot on the MIT campus Thursday, April 18, 2013. Authorities allege that the Boston Marathon bombing suspects were responsible. (AP Photo/Elise Amendola)
In a photo provided by Massachusetts Institute of Technology, MIT police pallbearers carry the casket of fallen MIT officer Sean Collier during a memorial service on the MIT campus in Cambridge, Mass., Wednesday, April 24, 2013. Collier was fatally shot on campus Thursday night, April 18, 2013. Authorities allege that the Boston Marathon bombing suspects were responsible. (AP Photo/Massachusetts Institute of Technology, Dominick Reuter)
Mourners bow their heads during a memorial service for fallen Massachusetts Institute of Technology campus officer Sean Collier at MIT in Cambridge, Mass. Wednesday, April 24, 2013. Collier was fatally shot on the MIT campus Thursday, April 18, 2013. (AP Photo/Elise Amendola)
Mourners bow their heads during a memorial service for fallen Massachusetts Institute of Technology campus officer Sean Collier at MIT in Cambridge, Mass. Wednesday, April 24, 2013. Collier was fatally shot on the MIT campus Thursday, April 18, 2013. (AP Photo/Elise Amendola)
A Revere, Mass. police captain holds his cap while entering a memorial service for fallen Massachusetts Institute of Technology police officer Sean Collier, in Cambridge, Mass., Wednesday, April 24, 2013. Collier was fatally shot on the MIT campus Thursday, April 18, 2013. Authorities allege that the Boston Marathon bombing suspects were responsible. (AP Photo/Steven Senne)
