The National Transportation Safety Board is looking into what may have caused the deadly midair collision over the Potomac between a Black Hawk helicopter and a regional jet. The NTSB says that data from the chopper’s black box shows that it was flying too high but the pilots may not have known it because of potential altimeter malfunctions.

The NTSB said Army Black Hawk crew may not have heard a message to "pass behind" the D.C.-bound passenger plane before the Jan. 29 midair crash over the Potomac River.