Hartsfield Jackson International airport in Atlanta, Georgia, USA

The Federal Aviation Administration (FAA) has launched an investigation into the world’s busiest airport.

According to The Atlanta Journal-Constitution, the FAA informed the city of Atlanta on July 25 that it’s looking into whether Hartsfield-Jackson Atlanta International Airport unlawfully diverted revenue to the city to cover legal fees.

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The penalty could be the withholding of federal funds.

Documents show that city officials have already acknowledged that invoices totaling more than $100,000 “were wrongly paid from airport funds” and that it reimbursed the airport for the payments, AJC reports.

FAA regulations bar the use of airport revenue for any purposes other than an airport’s capital or operating costs with the aim of preventing local governments from using airports as a means to funnel away their money and ensure that federal airport grants support actual airport projects.

Citing an earlier AJC article, the FAA notice points out that the city “may have made ineligible payments to law firms from airport revenue accounts.” The FAA also claims that the city “on multiple occasions, refused to comply with the FAA requests for un-redacted legal fees invoices.”

The city of Atlanta has 30 days to respond to the notice of investigation and the subpoena.

“In all of its communications, the city has been clear about its desire to cooperate and has taken steps to do so while maintaining its attorney-client privilege and work product protections,” Atlanta Mayor Keisha Lance Bottoms’ spokesman Michael Smith said in a statement via AJC. “Waiving our asserted privileges is an extraordinary and unreasonable request.”

“We look forward to working cooperatively with the FAA to produce the requested documents.”