An air traffic control supervisor at Reagan National Airport allowed a controller to leave early before a deadly midair crash, leaving one controller to handle both plane and helicopter traffic. The National Transportation Safety Board is investigating the crash, with staffing levels being just one aspect of the overall investigation. The NTSB is conducting a thorough investigation into the tragedy, which claimed the lives of 67 people, including young figure skaters, coaches, and parents. The agency has established working groups to investigate different areas connected to the accident, and a recovery operation is underway.