Delta Air Lines is under investigation by the U.S. Department of Transportation’s Office of Aviation Consumer Protection as the airline scraps hundreds of flights for a fifth straight day after a faulty software update from cybersecurity company CrowdStrike took down Microsoft systems around the world. 

While the outage impacted many businesses, from retailers to airlines, most have regained their footing and resumed regular operations. As of 8 a.m. Eastern on Tuesday, however, Delta had canceled 415 flights, far exceeding cancellations by any other U.S. airline, according to data from flight tracker FlightAware.

In a statement sent to CBS News on Tuesday, the Transportation Department said it “is investigating Delta Air Lines following continued widespread flight disruptions and reports of concerning customer service failures.”

The airline is continuing to struggle with the aftermath of the outage, causing frustrations for travelers trying to get home or go on vacation. Some have opted to pay for pricey tickets on other airlines in order to get to their destinations, according to CBS Boston. 

In a Monday statement, Delta said its employees are “working 24/7” to restore its operations, but CEO Ed Bastian also said it would take “another couple days” before “the worst is clearly behind us.” Other carriers have returned to nearly normal levels of service disruptions, intensifying the glare on Delta’s relatively weaker response to the outage that hit airlines, hospitals and businesses around the world.

“I’m so exhausted, I’m so upset — not because of the outage, but the lack of transparency,” Charity Mutasa, who was delayed by a day trying to get a Delta flight back to Boston from Dallas, told CBS Boston.

Another traveler, Matthew Dardet, told CBS Boston he ended up paying three times his original Delta ticket price for a seat on JetBlue after his flight to Florida was canceled multiple times. He was traveling to make it to his grandfather’s 82nd birthday.

Delta has canceled more than 5,500 flights since the outage started early Friday morning, including more than 700 flights on Monday, according to aviation-data provider Cirium. Delta and its regional affiliates accounted for about two-thirds of all cancellations worldwide Monday, including nearly all aborted flights in the United States.

United Airlines was the next-worst performer since the onset of the outage, canceling nearly 1,500 flights. United canceled 40 flights on Tuesday morning, FlightAware’s data shows.

Crew-tracking software

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 are the percentage of sold seats on each flight.

Meanwhile, the failures from CrowdStrike and Delta are drawing the attention of regulators and lawmakers. U.S. House leaders are calling on CrowdStrike CEO George Kurtz to testify to Congress about the cybersecurity company’s role in the tech outage. 

Transportation Secretary Pete Buttigieg spoke to Delta CEO Ed Bastian on Sunday 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 those customers who don’t want to be rebooked on a later flight.

“No one should be stranded at an airport overnight or stuck on hold for hours waiting to talk to a customer service agent,” Buttigieg said. He vowed to help Delta passengers by enforcing air travel consumer-protection rules.

—With reporting by the Associated Press.