By Karen Anderson, WBZ-TV

SOMERVILLE (CBS) – Four months after Sean Collier was allegedly killed by the Boston Marathon bombing suspects, he was sworn in as a Somerville Police officer.

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“He should be here, it’s as simple as that,” said Sean’s friend Brandon Kelly.

“It was his dream to become a Somerville police officer,” Kelly said. “It was everything he wanted out of life, and to finally achieve that it was nice.“

Sean Collier

Sean Collier

On Thursday night, the city posthumously awarded Collier with Somerville Police badge number 310.

“Badge 310 will forever be dedicated to Officer Collier,” Somerville Mayor Joe Curtatone said.

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“In death Sean achieved his dreams and his memory like his badge, 310 will live with the Somerville Police Department eternally,” said Somerville Police Chief Thomas Pasquarello.

Collier was the MIT officer who investigators say was killed by Dzhokhar and Tamerlan Tsarnaev before their shootout with police in Watertown.

“Sean loved being a police officer,” said Dep. Chief Michael Cabral. “He fulfilled his dream and he made it, he did and he died doing what he loved most.”

Collier’s classmate and friend MBTA officer Richard Donohue, who was also shot that night, wanted to attend.

“He loved being a police officer, he loved the city of Somerville, this is something he always wanted so I am glad to be here tonight, it’s an honor,” said Donohue.

After a vote, Badge 310, now retired, was presented to the Collier family.

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Sean’s brother Andrew said, “to police and everyone remember to be strong, be Collier Strong, do it for MIT officer 179 and Somerville officer 310.”