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.

Incorrect altitude readings on the Army helicopter that collided with a passenger plane over Washington, D.C., in January contributed to the aircraft getting too close, but air traffic controllers warned about the hazards helicopter traffic presented years before the crash.