US budget carrier Southwest Airlines resumed operations after a technical fault delayed thousands of flights.
Much of the airline’s network ground to a halt from 9.45am local time on Tuesday, with flights resuming at 11.10am.
The FlightAware website calculated that 2,405 Southwest flights – more than half of the total – were delayed.
Dallas-based Southwest said flights had returned after “temporarily pausing” to work through “data connection issues”.
The largest US domestic carrier added: “We offer our heartfelt apologies to customers whose journey with us today might be delayed.”
Affected passengers were offered the ability to rebook to travel within 14 days without incurring extra charges.
The US Federal Aviation Administration tweeted that Southwest had “experienced a technical issues with one of its internal systems”.
The FAA said it paused departures at the airline’s request to enable the issue to be resolved.
But many passengers took to social media to complain and raise concerns about the status of their flights.
Southwest had blamed staff shortages for 16,700 cancelled flights over the Christmas holiday peak travel period, leading to an investigation by the US Department of Transportation.
The US DoT said last week: “More than half (51.7%) of the complaints received in December 2022 were against Southwest Airlines (8,729 complaints) and concerned travel problems that stranded millions.
“DoT is in the initial phase of a rigorous and comprehensive investigation into this incident and has made clear to Southwest that it must provide timely refunds and reimbursements and that the department will hold the airline accountable if it fails to do so.
“DoT is also probing whether Southwest engaged in unrealistic scheduling of flights which under federal law is considered an unfair and deceptive practice. DoT will leverage the full extent of its investigative and enforcement powers to ensure consumers are protected.”