Now, just stop and think for a second. If you had just bombed the marathon, and needed to hijack a car and plan to let the driver live, why would you announce you were the bombers and that you had killed a cop, knowing that the driver would them immediately report that to police?
The answer is that you would not.
So, my theory: Operatives shot and killed the police officer in his car to scare the Boston police into thinking the two kids they were chasing were cop-killers, so that the police would shoot first and ask questions later. Then these operatives hijacked the car, identified themselves as the bombers and claimed credit for the cop killing to make sure the police "knew" the kids were the killers even though there was no evidence available at the time who shot the police officer.
Could it be the same Tamerlan Tsarnaev that Federal authorities and Boston police say was shot and died at the scene of a shootout?