More than 1,000 flights were delayed or canceled on Wednesday and Thursday after an IT outage.

The Unimatic system is used by airline United Airlines to store flight information, with the system going down on Wednesday for reasons as yet unknown. The company confirmed that the outage was not due to a cybersecurity issue.

“The underlying technology issue has been resolved, and, while we expect residual delays, our team is working to restore our normal operations," the company said in a Thursday statement.

By Friday, it said that flights had fully resumed. During the peak of the outage, flights were grounded across several major United hub airports, including Newark, Denver, Houston, and Chicago.

The US transportation secretary Sean Duffy clarified on X that the problem was solely with United’s operations and was unrelated to the broader air traffic control system.

In June, Alaska Airlines grounded its flights after an unspecified IT outage.