The U.S.’s deadliest air crash in more than two decades serves as a reminder of the yearslong shortage of air-traffic controllers and the safety risks to aircraft taking off and landing at the nation’s busiest airports.

Since the midair crash in Washington, D.C., there have been questions about air traffic controller staffing. According to the latest FAA data available, more than 90% of air traffic control towers are understaffed.

The new secretary of transportation, Sean Duffy, appeared to walk back an enticement to retire, saying F.A.A. safety workers were exempt from the offer for federal workers.