American Airlines Group Inc. was ordered to pay a $4.1 million fine, the largest such penalty to date, for allowing aircraft to sit on the ground for three hours or more without giving passengers a chance to exit.

The US Transportation Department on Monday said the action was part of an increase in enforcement starting last year after Secretary Pete Buttigieg began denouncing carriers for causing lengthy delays and canceling flights.

“This is the latest action in our continued drive to enforce the rights of airline passengers,” Buttigieg said in a statement. “Whether the issue is extreme tarmac delays or problems getting refunds, DOT will continue to protect consumers and hold airlines accountable.”

US law prohibits domestic flights from sitting on airport tarmacs for more than three hours without pulling planes back to gates to allow people to deplane. The DOT alleged that American violated the rules on 43 domestic flights between 2018 and 2021. A total of 5,821 passengers were aboard the flights, the agency said.