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STORY: The pain of last week’s massive IT outage isn’t over, for airlines or U.S. travelers.

Over the weekend, Delta was struggling to get back to normal.

On Sunday it cancelled around 1,000 more flights - on top of the 3,500 it had already halted.

Many of the services it did operate were delayed.

All that has stranded thousands of passengers all over the U.S.

One traveler in Atlanta - Delta’s home base - said she'd been through chaos:

"You can't even rent any cars. I called all of them. We barely got a hotel room last night. We were lucky that we even got one. I called five different hotels before I was able to get a hotel room for me and my husband."

Delta faces ongoing problems caused by the outage’s impact on its crew tracking system.

It hasn’t been able to say when normal operations will resume, and has already cancelled some Monday flights.

The problem originated with a software update from global cybersecurity firm CrowdStrike.

That triggered problems for millions of Microsoft users around the world, including many airlines.

Other carriers have now mostly recovered, though United cancelled 9% of its services on Sunday.

Transport Secretary Pete Buttigieg spoke with Delta chief Ed Bastian on Sunday.

An official says he reminded the airline boss of Delta’s responsibility to customers, and the department’s enforcement powers.