U.S. airlines were notified this week that an investigation is underway into whether they complied with requiring flight cuts at 40 major airports during the record , 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 . The airlines have 30 days to provide documentation showing they complied with the order, the agency said Friday in a statement.

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