Delta Air Lines has started to recover from an operational meltdown that left hundreds of thousands of travelers stranded around the country over the last five days. But the airline is drawing new scrutiny from federal lawmakers as an investigation into the matter begins.

Atlanta-based Delta had more than 50 flight cancellations Wednesday as of about 4:30 p.m., or about 1% of its flights. That’s a significant improvement from the more than 1,000 flight cancellations a day it had Friday, Saturday, Sunday and Monday, and its more than 500 cancellations Tuesday.

At Hartsfield-Jackson on Wednesday morning, baggage belonging to passengers whose flights got disrupted over the last several days remained lined up alongside baggage carousels.

Some travelers still spent the night at Hartsfield-Jackson after flight cancellations Tuesday night.

“I’m upset,” said traveler Tanya Fantasia, who got stuck in Atlanta Tuesday night on her way to Huntsville, Alabama, from Melbourne, Florida. She spent the night walking around the airport, “trying to stay awake because you can’t sleep here. ....I’m stressed out.”

But there were other signs that the world’s busiest airport was returning to its normal busy schedule of flights, with passengers moving through Delta check-in areas and hundreds of passengers lined up for security screening.

Delta CEO Ed Bastian said in an email to customers Wednesday morning that he expected cancellations Wednesday to be “minimal.”

“Thursday is expected to be a normal day, with the airline fully recovered and operating at a traditional level of reliability,” said Bastian, who apologized to customers.

Meanwhile, Delta faces growing criticism and an investigation by the U.S. Department of Transportation into the mass cancellations and its treatment of passengers.

U.S. Sen. Maria Cantwell, D-Washington, who chairs the Senate commerce committee, wrote a letter to Bastian on Tuesday telling him her committee would conduct oversight into the underlying reasons for the outage. She wrote that the committee would seek answers from the airline industry about “ensuring redundancy to prevent future widespread outages as the global and national impacts on air travel and the flying public are far too important.”

She also emphasized what the law requires of airlines, including providing “real-time assistance from customer service agents of air carriers without an excessive wait time, particularly during times of mass disruptions” and ensuring passengers’ right to a refund for canceled or significantly delayed flights.

“While the technology outage was clearly not caused by Delta or any airline, I am nevertheless concerned that Delta is failing to meet the moment and adequately protect the needs of passengers,” Cantwell wrote.

Delta’s entire network was hobbled by a faulty security update to Windows systems initiated by cybersecurity company CrowdStrike. The situation crippled airlines around the world, but its impact on Delta was particularly severe.

Travelers have been sleeping at the Atlanta airport and stranded at airports around the world since the meltdown began Friday with a technology outage that affected many Microsoft users. About 60% of Delta’s most critical systems to run the airline are Windows-based and were rendered inoperable early Friday morning, according to the airline.

U.S. Rep. Rick Larsen, D-Washington, who is a ranking member on the House transportation committee, issued a statement Tuesday calling the response by some airlines to the meltdown “unacceptable.”

He added that “any passengers experiencing these kinds of extreme flight delays or cancellations are entitled to be made whole, and I fully expect that to happen here.”

U.S. Transportation Secretary Pete Buttigieg during a news conference on Tuesday on the Delta operational meltdown said he was very concerned about “people being on hold for hours and hours trying to get a new flight, people having to sleep on airport floors.”

“I directly communicated to Delta’s CEO our expectation that Delta will live up to their passenger service commitments,” he said. “Clearly there is something unique to Delta that requires specific attention.”

Buttigieg said the DOT has received more than 3,000 complaints “specific to this situation and Delta.” He added that he has heard “really unacceptable accounts of a line with 100 passengers waiting to talk to one customer service agent who’s there to handle their vouchers and refunds.”

On Tuesday night, Bastian flew to Paris for the Olympics. Delta is the official airline of Team USA. Bastian’s departure for Europe drew criticism from the Delta organizing committee of the Association of Flight Attendants union, which posted on its website: “Ed and several board members traveled to Paris instead of being on the front lines leading us through this turmoil.”

Delta issued a statement saying: “Ed delayed this long-planned business trip until he was confident the airline was firmly on the path to recovery.”

“Ed remains fully engaged with senior operations leaders,” the company said.