UPDATE: United Airlines Resumes Domestic Flights After Temporary Ground Order

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CHICAGO (CNN) — United Airlines resumed operations Sunday (Jan. 22) night after a computer problem temporarily grounded all domestic mainline flights, two sources familiar with the incident told CNN.

“The ground stop has been lifted,” the airline announced on Twitter. “We’re working to get flights on their way. We apologize for the inconvenience to our customers.”

Earlier, United spokeswoman Maddie King told CNN that the flights were grounded due to an IT issue.

“We have issued a ground stop for all domestic mainline flights due to an IT issue,” King said. “We are working as quickly as possible to resolve this issue and get our customers to their final destinations. We apologize for the inconvenience to our customers.”

Mainline flights are those operated by an airline’s main operating unit, not regional alliances or subsidiaries.

Other computer glitches have hit airlines.

In October, United flights were delayed for hours because of a computer problem, causing a social media firestorm.

United said it experienced an issue with its weight reporting system that was later resolved.

In September, British Airways suffered a computer problem that caused delays and long lines at airports in the United States and Europe.

In August, Delta was forced to cancel around 2,000 flights after a major system failure. That came only a few weeks after an outage caused more than 1,000 cancellations at Southwest Airlines.