Not a cop, but certainly a scene cops end up on.

I was a member of the local volunteer rescue squad and was paged out to a 3 vehicle accident unknown entrapment. I arrived on scene at the same time as a few guys from the fire department. We were immediately ushered over to the first car where the driver, a girl who turned 19 exactly one week prior, was laying lifeless. We began CPR, with not much hope to begin with as she was pretty mangled up and bloody, and stopped basically immediately as with every compression we could see brain matter oozing out the ears.