“As part of this investigation, DOT has conducted on-site audits of Southwest’s refunds and reimbursements records, met with numerous Southwest officials to assess the causes of Southwest’s system-wide winter holiday breakdown, and is examining tens of thousands of pages of documents,” Arndt said.

Southwest Airlines spokesperson Brandy King said the airline will continue to cooperate with the federal investigation.

“It remains our understanding that the DOT is continuing to take the lead on this matter,” King said in an email. “We have not received any inquiries from the Department of Justice but remain ready to cooperate with all interested government agencies.”

The Justice Department did not immediately respond to a request for comment.

Background: DOT Secretary Pete Buttigieg announced his agency’s investigation into Southwest in January, focused on whether Southwest engaged in unrealistic scheduling that would be considered an unfair and deceptive practice under federal law. DOT also worked with Southwest to determine how many affected passengers were eligible for refunds under federal law.

Southwest CEO Bob Jordan previously said that “basically anybody” who dealt with flight issues between Dec. 24 and Jan. 2 was “refunded or [we] gave you a travel credit.” Jordan said that many Southwest passengers were given free tickets and rewards points, along with reimbursing customers who had to buy another airline ticket, stay in a hotel, buy a meal or hire a taxi.

In response to the December meltdown, Southwest investigated itself with the help of an outside consulting firm and promised to upgrade its winter infrastructure and implement software updates.

But Senate Commerce Chair Maria Cantwell (D-Wash.), whose panel oversees the airline industry, has continued to push Southwest for more details about how many passengers were involved, how many were issued cash refunds versus vouchers for future flights, how many were rebooked and when the airline plans to upgrade its internal systems that caused the debacle. Cantwell said Southwest has provided some information but not enough. Recently she has complained that Southwest was withholding details her office had requested.

And Southwest on Tuesday asked the Federal Aviation Administration to “pause” all flights for about 40 minutes due to what the airline said was “data connection issues resulting from a firewall failure.” More than 2,000 flights were delayed across the country, though cancellations were minimal.

Both Buttigieg and Cantwell said they are keeping close tabs on Southwest following the latest inconvenience for travelers.

“This is another demonstration that Southwest Airlines needs to upgrade their systems,” Cantwell said in a statement. Buttigieg added that DOT is “watching to ensure that Southwest Airlines takes care of all passengers who were affected by their technical problems.”

What’s next: Southwest reports its latest quarterly earnings next week.