The 27-year-old Massachusetts Institute of Technology (MIT) cop Sean Collier was shot execution style at point-blank range as the Tsarnaev brothers attempted to steal his gun.

Investigators have revealed that the Boston terrorists, brothers Tamerlan and Dzhokhar Tsarnaev, originally from Chechnya, only had one real gun and one pellet gun. Because Collier’s holster had a locking system they couldn’t get the gun out. Collier was shot at point-blank range in the head as he sat in his patrol car.

One of the first responders to these shots fired at MIT was Richard D Donohue JR, who had attended Police Academy with Collier. He was shot later on Thursday evening during a shootout with the Tsarnaev brothers in Watertown. Tamerlan was killed in the shootout but his teenage brother was not apprehended until Friday evening. He remains in hospital.

On Wednesday Collier was laid to rest at a church in Stoneham, Massachusetts. A private funeral Mass was held for the officer on Tuesday.

Bagpipes serenaded the 4,000 mourners including Vice President Joe Biden, MIT students, faculty and staff, and law enforcement officials from across the nation. The line of mourners stretched back a half-mile as they made their way through the tight security for the memorial service.

The Vice President told the Collier family that no child should die before their parents but that in time the grief would lose some of its sting.

He said, “The moment will come when the memory of Sean is triggered and you know it's going to be okay. When the first instinct is to get a smile on your lips before a tear to your eye."

In a statement released by Collier’s family they said, “Our only solace is that Sean died bravely doing what he committed his life to - serving and protecting others.”

Collier had been an MIT cop for just under a year. He had impressed students and his colleagues with his contagious enthusiasm, be it offering students rides or volunteering for extra duties.

MIT Police Chief John DiFava said, “Just the other day - and I still have it on my computer -  he asked me if I would have a problem if he approached the homeless shelter to see if he could become a member of the board of directors, so that he could work with those people down there and try to mitigate problems before the problems develop.

“The kid was the real deal.”

The 27-year-old enjoyed mountain climbing, training young boxers and playing kickball on a team called Kickhopopotamus. According to the Daily Mail, Collier had been a volunteer at the boxing gym where Tamerlan was a member and is believed to have helped with his training.

He was a Wilmington native and a graduate of Salem State University. He is survived by his parents and five siblings.

Here’s the CBS News update on his shooting:

Here’s some footage from the memorial service: