The two helicopters crashed while covering a police pursuit, killing at least 4 people.
The cameras from both helicopters were pointed at the ground, so there is no video of the accident.
Video from KNXV-TV (ABC15, PHX): In this video, you can hear the pilot go "Oh geez" and then everything screws up.
It collided with Channel 3 from Phoenix.
Another helicopter's emotional reaction after seeing the accident: http://www.liveleak.com/view?i=882_1185574862
You've probably seen both videos before, on the news. But anyway.