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.

During the hearings, the NTSB was told that the Army helicopter never heard the command from the air traffic controller to "pass behind the CRJ" as the transmission was stepped on. It was also revealed that the plane's pilots were not warned by the controller that there was a helicopter nearby or cleared to fly near the helicopter.

The deadly mid-air collision at Reagan Airport in January was years in the making, the operations manager of the DCA air traffic control tower on the night of the accident told the National Transportation Safety Board on Friday.