Last week, the NTSB held three days of hearings looking into what led up to the fatal crash between a plane and a helicopter in January.

The deadly mid-air collision at Reagan Airport in January was years in the making, the operations manager of the DCA air traffic control tower on the night of the accident told the National Transportation Safety Board on Friday.

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,

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

The NTSB is now reviewing three days of testimony from last week’s public hearings into the Jan. 29 midair collision near Reagan National Airport.

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

The deadly mid-air collision at Reagan Airport in January was years in the making, the operations manager of the DCA air traffic control tower on the night of the accident told the National Transportation Safety Board on Friday.