Safety investigators probing January’s midair collision between an American Airlines Group Inc. regional jet and a US Army Black Hawk near Washington found a key instrument on the helicopter was displaying inaccurate altitude data that could have led the pilots to think they were flying lower than they actually were.

Federal investigators revealed Wednesday that the Black Hawk helicopter involved in the deadly DCA crash may have misreported its altitude by more than 100 feet, a flaw the Army had long known about but never formally documented.