The NTSB concluded three days of investigative hearings into the collision between an Army helicopter and a passenger plane that killed 67.

Investigators so far have highlighted several factors that may have contributed to the collision, which left 67 people dead.

A three-day National Transportation Safety Board hearing on the deadliest U.S. aviation accident in decades dug into problems with altimeters, chopper routes and the busy Washington, D.C., airspace.