The BBC's Analysis Editor Ros Atkins on the unanswered questions around a fatal crash between a commercial airplane and a military helicopter near Washington DC.

Sixty-seven people are presumed dead after a passenger plane on approach to Reagan National Airport near Washington, DC, collided Wednesday night with a US Army helicopter midair, sending both aircraft into the Potomac River below,

Sen. Tammy Duckworth, an ex-Army Black Hawk pilot, said communications between airline and military pilots usually go through air traffic control.