The National Transportation Safety Board, during hearings on Thursday, grilled officials from the Army, air traffic controllers and members from the Federal Aviation Administration over protocols following the January deadly air collision between a Black Hawk helicopter and a passenger jet approaching Ronald Reagan Washington National Airport (DCA.

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.