At their closest points, helicopters and planes routinely flew within 75 feet near the airport, according to the NTSB.

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

The National Transportation Safety Board questioned witnesses Friday on collision avoidance technology and organizational systems to manage risk. It is the third and final day of investigative hearings probing January’s midair collision between a US Army Black Hawk helicopter and an American Airlines regional jet,