How an Army Black Hawk helicopter collided into an American Airlines jetliner in a deadly crash over the Potomac River Wednesday night is still unknown, but questions are emerging, including the altitude of the military helicopter.

An American Airlines plane carrying 60 passengers and four crew members collided with an Army Black Hawk helicopter outside Reagan National Airport near Washington, D.C. Wednesday night. A D.C. fire official said Thursday that “we don't think there are any survivors from this accident" and "we are switching from a rescue operation to a recovery operation.