The National Transportation Safety Board will question witnesses Thursday about air traffic control training, a key part of the investigation into January’s deadly midair collision at Ronald Reagan Washington National Airport.

The NTSB is holding three days of hearings to uncover insights into what caused the fatal crash between the passenger jet and the army helicopter over Ronald Reagan National Airport in January.

Investigators have found that a helicopter involved in a deadly crash with a passenger plane over Washington was flying above its height limit