SOMERVILLE, Mass. – The Massachusetts Institute of Technology police officer slain just days after the Boston Marathon bombing has been posthumously sworn in as a Somerville officer.

Authorities allege the marathon bombing suspects shot and killed Sean Collier on April 18 to get his gun.

Collier’s brother spoke for his family at a ceremony at Somerville City Hall Thursday night. Boston’s WFXT-TV Fox25 reports he asked those who attended to be “Collier strong,” echoing the “Boston Strong” bombing recovery theme.

Among those attending was MBTA officer Richard Donohue, a friend of Collier’s wounded in a police shootout with the bombing suspects.

Collier was assigned badge number 310, which was then retired in his honor.

The 27-year-old had accepted a position in Somerville before he was killed and was just weeks away from starting his new job.