The Federal Aviation Administration this week told airlines it will investigate whether they complied with orders from the Trump administration during the record-long government shutdown to cut flights.
The orders came in November after the shutdown had been going for a month and airports were facing shortages of air traffic control workers.
The emergency order affected 40 major airports in the U.S. and fluctuated between cuts of 3% to 6% for each airline before the shutdown ended on Nov. 12.
In a letter sent Monday to U.S. airlines, the FAA warned that they could face $75,000 fines for each flight over the allotted limit during the shutdown.
Airlines have 30 days to prove they complied with the required cuts.
Air traffic controllers, like most other government workers, weren’t paid during the 43-day shutdown, and many missed work, sparking safety concerns.
The FAA lifted the restrictions Nov. 16, four days after the shutdown ended.
Despite the shutdown still being in effect Nov. 14 — when 6% flight cuts were required — only 2% of flights were actually cut, according to Cirium, a flight analytics firm.
The cuts also had a major financial impact on airlines, with Delta reporting that it lost $200 million between Nov. 7 and Nov. 16 when the order was in effect.
More than 10,000 flights were canceled in the U.S. during the nine-day period.
The Associated Press contributed to this report.