The National Transportation Safety Board will hold three days of hearings on the January midair collision that killed 67 people.

Investigators have found that a helicopter involved in a deadly crash with a passenger plane over Washington was flying above its height limit. The helicopter's altitude-measuring instrument was also inaccurate.

The National Transportation Safety Board on Thursday questioned the failure of an air traffic controller to issue a warning about a potential collision in the moments before a crash between an Army helicopter and an American Airlines regional jet that killed 67 people.