Investigators have said the helicopter crew was wearing night-vision goggles that would have limited their peripheral vision. PHOTOS: What we know about the deadly air crash between a passenger ...

The NTSB issued its preliminary report on the midair collision between a passenger plane and helicopter over Washington, D.C., in January.

Investigators previously said the helicopter may have had inaccurate altitude readings in the moments before the crash, and the crew may not have heard key instructions from air traffic controllers.