The cop that saw the gunman on the rooftop, climbed up but was hanging on by both hands. Gunman turned on him with the rifle so he let go and dropped about 8 feet to the ground below...
https://www.youtube.com/watch?v=ZOn7v-AdPjc&t=11s
now it makes sense that he couldnt take out the shooter since he was hanging on with both hands.After that however you gotta wonder what happened,how and why.