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.

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,

A second vote will be scheduled for September and will require the green light from nine members of the council.