An NTSB investigation into a fatal helicopter-airplane collision in D.C. revealed a faulty helicopter altimeter and prior warnings to the FAA about helicopter traffic dangers. The NTSB criticized the FAA's inaction despite safety concerns and near misses.

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.

Three days of investigative hearings on the deadly midair collision over Washington in January begin Wednesday and aim to reveal new insights into what caused the crash between a passenger plane and an Army helicopter that killed 67 people.