From the Associated press reports;
"Less than 30 seconds before the collision, air traffic recordings show controllers asking the helicopter if it has the plane in sight and instructing it to pass behind the landing aircraft.
A crewmember on the helicopter replies that “the aircraft is in sight” and requests “visual separation” with the incoming plane, allowing it to fly closer than may otherwise be allowed if the pilots didn’t see the plane.
The controllers approved the request.
About 20 seconds later, a commotion is heard on the audio, and seconds after that, controllers begin diverting aircraft away from the disaster scene."
WUSA9 in DC is live broadcasting a press conference at present. (0100 EST)