A Bombardier CRJ-701ER passenger jet and a Black Hawk helicopter collided in midair Wednesday and crashed into the Potomac River near Reagan National Airport near Washington, D.C., and President Trump confirmed Thursday that there were no survivors.

Experts say both the pilots aboard American Airlines Flight 5342 and the military pilots in the Black Hawk would have been used to navigating the complex airspace, and the head of the nation’s air traffic controllers union said it’s too soon to speculate on a cause. The National Transportation Safety Board is leading a probe into the collision.