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.

Friday was the final day of the National Transportation Safety Board's (NTSB) three-day hearing on the deadly plane crash at Reagan Airport that took the lives of 67 people in January.