WASHINGTON (AP) — United Airlines has been fined $1.9 million over 25 flights in which the plane sat on the ground for several hours, the largest fine imposed by the government for such long delays.

The Transportation Department said Friday that the incidents occurred between December 2015 and February of this year. More than 3,200 passengers were trapped on planes without being given a chance to disembark, the department said.

In a consent order, United said most of the delays involved diversions caused by severe weather, when the airline is focused on making sure that planes land safely. The airline said it was a small number of the nearly 8 million operated by United and its United Express affiliates over the five-year span of the violations.

United was ordered to pay $950,000. The airline was given credit of $750,000 for compensating passengers on delayed flights and $200,000 for the cost of developing a tool to improve management of diverted flights.

Federal rules require airlines to give passengers a chance to return to the terminal if a plane on a domestic flight sits on the ground for at least three hours — four hours in the case of international flights. Exceptions are allowed for safety, security, or air traffic control problems.

The longest delay, more than five hours, occurred in April 2016 after an international flight bound for Houston diverted to New Orleans. Passengers were only given a chance to deplane after pilots exceeded their legal working day and had to be replaced with another crew, according to a consent order.

Five delays occurred on the same day in April 2019, when a winter storm snarled Chicago's O'Hare Airport, prompting flights to divert to Dane County Regional Airport in Wisconsin.