(Reuters) - Delta Air Lines reported sharply fewer cancellations Wednesday as the U.S. airline looks to return to normal after cancelling more than 6,000 flights since Friday.

The Atlanta-based carrier said as of 6:30 a.m. ET (1030 GMT) it had canceled just 47 flights - only 1% of its flights - compared with 511 on Tuesday and 1,160 on Monday. Delta said late Tuesday it was "seeing solid day-over-day progress across operating metrics that the entire team should be proud of".

A software update by global cybersecurity firm CrowdStrike triggered system problems for Microsoft customers, including many airlines, on Friday. But disruptions at other major U.S. carriers had largely subsided after Friday but persisted at Delta.

The U.S. Transportation Department opened an investigation Tuesday into the massive Delta disruptions that impacted more than 500,000 passengers and stranded people across the United States.

Transportation Secretary Pete Buttigieg said the investigation is focused on "how this breakdown could have happened, and how it could be that days after the other airlines are back to normal" as well Delta customer service resources.

"We require a basic level of customer service," he added.

The issue has left customers fuming. Many complained they waited hours for assistance as the airline's helplines were overwhelmed. Some were forced to rent cars, driving hundreds of miles to get to destinations, while others said they would have to wait days for new flights.

Representative Rick Larsen, the top Democrat on the House Transportation Committee, said he will introduce legislation to boost the operational resiliency of airlines in the near future. "The slow response by some airlines to this meltdown has been unacceptable," Larsen said.

Senate Commerce Committee chair Maria Cantwell said she is concerned Delta is not complying with passenger rights obligations under a new law.

In December, Southwest Airlines agreed to a record-setting $140 million civil penalty over the 2022 holiday meltdown that led to 16,900 flight cancellations and stranded 2 million passengers, resolving a USDOT investigation.