Delta flight cancellations being investigated by U.S. Department of Transportation

The Department of Transportation is investigating Delta Air Lines over flight cancellations related to a global outage caused by a faulty software update last week affecting businesses worldwide. 

U.S. DOT Secretary Pete Buttigieg said Tuesday morning that the agency is probing Delta to make sure the company is "following the law and taking care of its passengers during continued widespread disruptions."

Delta is struggling to resume normal services after a worldwide IT outage by cybersecurity firm CrowdStrike on July 19. 

As of Tuesday morning, Delta canceled more than 400 flights, representing the most cancellations by any other U.S. airline, according to data from flight tracker FlightAware. 

RELATED: Delta struggles to recover from global tech outage, stranding tens of thousands

How many flights has Delta canceled since last week's global outage?

The Atlanta-based airline has canceled over 5,500 flights since the outage started on July 19, including at least 700 flights canceled on Monday, according to aviation data provider Cirium. The Associated Press reported that Delta and its regional affiliates accounted for about two-thirds of all cancellations worldwide Monday, including nearly all the ones in the United States.

Buttigieg spoke to Delta CEO Ed Bastian on July 21 about the airline’s high number of cancellations since Friday. Buttigieg said his agency had received "hundreds of complaints" about Delta, and he expects the airline to provide hotels and meals for travelers who are delayed and to issue quick refunds to customers who don’t want to be rebooked on a later flight.

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Bastian, the Delta CEO, said in a message to customers Sunday that the airline was continuing to restore operations that were disrupted. One of the tools Delta uses to track crews was affected and could not process the high number of changes triggered by the outage.

"The technology issue occurred on the busiest travel weekend of the summer, with our booked loads exceeding 90%, limiting our re-accommodation capabilities," Bastian wrote. Loads refer to the percentage of seats on each flight that had been sold.

Delta has offered waivers to make it easier for customers to reschedule trips.

The Associated Press and FOX Television Stations national reporter Chris Williams contributed to this report. This story was reported from Washington, D.C.