The U.S. Department of Transportation has filed a complaint against Delta Air Lines, saying the airline violated requirements to disclose when flights are operated by partner carriers.

The issue deals with so-called “code-share” marketing partnerships between carriers, which allow one airline to market flights operated by another airline. The DOT’s aviation enforcement office alleges Delta telephone reservations agents failed to disclose code-share information during calls with prospective consumers in 2013 and 2014, violated a 2010 cease-and-desist order, and could be fined up to $660,000.

The DOT filed the complaint after settlement negotiations were unsuccessful. Delta said it did not settle because it believes it complied with the disclosure requirements, adding that reservations agents are trained on the necessity of code-share disclosures to customers before they buy a ticket.