WASHINGTON — The Transportation Department is investigating scores of lengthy tarmac delays at the Atlanta airport in December, when a snowstorm followed by a power outage more than a week later disrupted thousands of flights.

The snowstorm on Dec. 8 delayed 14 flights on the tarmac for more than three hours while waited to depart Hartsfield-Jackson Atlanta International Airport, the department’s Air Travel Consumer Report announced Thursday.

A fire Dec. 17 knocked out power lines that serve the airport, which halted flights most of the day. The outage led to 77 lengthy delays for domestic flights and 19 delays of at least four hours for international flights, according to the report.

The department is investigating all of the lengthy delays. Hefty fines are possible when planes don’t allow passengers to exit within three hours for a domestic flight or four hours for an international flight.

Delta Air Lines, which is based in Atlanta, suffered disproportionately. The airline estimated in January that it lost $60 million in pre-tax income from the storm and power outage.

The airline typically prides itself on its completion factor. But Delta had the highest rate of cancelled flights among the dozen airlines that report to the department, with 2.9% of flights cancelled.

Delta CEO Ed Bastian has said the airline would try to recover losses from the outage from the airport and the city of Atlanta. But no resolution has been announced yet.

The six longest domestic delays for the month and the five longest international delays were all Delta flights, and all were on the day of the power outage.

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The six longest domestic delays were each more than six hours. The longest delays were 414 minutes for a flight arriving from Las Vegas, 411 minutes for a flight arriving from Rochester, N.Y., and 408 minutes for a flight arriving from Baton Rouge, La.

The longest international delays were each more than seven hours. The longest delays were 432 minutes for a flight from Nassau, Bahamas; 423 minutes for a flight from Toronto and 416 minutes for a flight from Panama City, Panama.