U.S. airlines were notified this week that an investigation is underway into whether they complied with an emergency order requiring flight cuts at 40 major airports during the record government shutdown, the Federal Aviation Administration said Friday.

The FAA warned in letters sent Monday that the airlines could face fines of up to $75,000 for each flight over the mandated reductions, which fluctuated between 3%, 4%, and 6%. The airlines have 30 days to provide documentation showing they complied with the order, the agency said Friday in a statement.

The 43-day shutdown that began Oct. 1 led to long delays as unpaid air traffic controllers missed work, citing stress and the need to take on side jobs. The FAA said requiring all commercial airlines to cut domestic flights was unprecedented but necessary to ensure safe air travel until staffing at its control towers and facilities improved.

After the shutdown ended Nov. 12, airlines seemed to anticipate that the FAA would lift or relax the restrictions. With the order still in place on Nov. 14 requiring 6% cuts, just 2% of scheduled U.S. departures that day were canceled, according to aviation analytics firm Cirium.

More than 10,000 flights were canceled between Nov. 7, when the order took effect, and Nov. 16, when the FAA announced it was lifting all flight restrictions. Delta Air Lines said Wednesday it lost $200 million, the first disclosure by a major airline regarding the shutdown’s financial impact.

Transportation Secretary Sean Duffy hasn’t shared the specific safety data that he and the head of the FAA said prompted the cuts, but Duffy cited reports during the shutdown of planes getting too close in the air, more runway incursions and pilot concerns about controllers’ responses.

Large hubs in New York, Chicago, Los Angeles and Atlanta were impacted by the cancellations. The FAA originally had a 10% reduction target.

Rio Yamat, The Associated Press