The Chicago-based airline also filed false reports to the government about mishandled bags and oversales, DOT alleged, and didn’t file timely reports about incidents involving the loss, injury or death of animals on flights.

The fine was specifically for the slow refunds, the DOT said.

“When passengers are owed a refund, they have the right to expect the airline to act promptly and give them their money back,” U.S. Transportation Secretary Anthony Foxx said in a statement.

Airlines must process refund requests within seven days when the ticket is purchased by credit card and 20 days when purchased by cash or check, the DOT said. During an on-site inspection at the airline’s headquarters in Chicago, the DOT found that between March and May of 2012, United failed to process more than 9,000 refund requests in a timely manner.

A United spokesman said in a statement, “We are committed to providing our customers the level of service they deserve and processing refunds in a timely manner and have since resolved these issues.”

The DOT did not fine United for the reporting errors because the airline disclosed it and took corrective action, the DOT said.

United underreported the number of mishandled passenger bags between January and October 2011 and the number of passengers it bumped from flights for each quarter of 2011.

“The underreporting made United’s ranking in these categories seem better than it actually was,” the DOT said. Such data is released monthly in the Air Travel Consumer Report.

Also during 2012 and 2013, DOT said United failed to file timely reports for a few incidents involving the death, injury or loss of animals on its flights.