US investigators have confirmed the military helicopter involved in a mid-air collision, was on a training flight

Sixty-seven people were on board both aircraft, with more than 40 already recovered from the Potomac River, with two debris fields for both vehicles identified.

A preliminary report has found air traffic control at the airport, for both planes and helicopters, had been combined earlier than usual prior to the crash.

National Transportation Safety Board investigator Todd Inman, says the helicopter's flight crew were also using night vision goggles.

He says they don't know if the night vision goggles were on at the time of the flight.