Transportation officials began the first of three days of investigative hearings Wednesday to find out what caused the deadly January plane crash that left 67 dead.

A test flight in May over the Potomac River found that the barometric altitude was consistently lower by about 100 feet than the radio and geometric altitudes over the water. The helicopter's barometric altimeter was also obscured during the flight, investigators said.

The U.S. Army helicopter involved in the midair collision over Ronald Reagan National Airport in January was flying higher than it was supposed to be when it crashed into an American Airlines flight that was about to land at the airport,