http://boston.cbslocal.com/2013/04/23/m ... ded-a-gun/“The original question is they walked up to that car and appeared they shot the officer in the head unprovoked, that it was an assassination. But why? How did that fit into their plan? The operating theory now in the investigation is they were short one gun. The older brother had a gun. They wanted to get a gun for the younger brother and the fastest and most efficient way they could think of doing it was a surprise attack on a cop, to take his weapon and go. Officer Collier had a locking holster, it’s like a three-way lock. If you don’t know how to remove the gun, you’re not going to get it out. There was apparently an attempt to yank it and they couldn’t get it and left. “
I guess they had no time or interest in a mandatory background check.