U.S. airlines were back in operation on Monday after a series of system-wide outages delayed flights and fired-up customer complaints on social media.

The Federal Aviation Administration said the problem was caused by the Aerodata airplane planning weight and balance program.

The agency issued a statement around 8.30 a.m. ET, saying the issue had been resolved.

American Airlines, Southwest Airlines and Delta Air Lines reported system-wide outages, with United Airlines saying it was unable to create paperwork for some time.

“A brief third-party technology issue that prevented some Delta Connection flights from being dispatched on time this morning has been resolved,” Delta said.

Other airlines also reported a series of delays as a result of the problem.

Southwest Airlines was the first carrier to report that the problem had been resolved and it would get travelers moving soon, before later adding customers should expect flight delays.

One Southwest passenger reported waiting on the tarmac in Dallas for 90 minutes after his 6 a.m. flight to New Orleans was delayed. The airline said after the systems resumed that the flight would arrive at 8.05 a.m.

Other passengers reported long wait times at airports and missed connections.

Reuters (Reporting by Sanjana Shivdas in Bengaluru and David Shepherdson in Washington; editing by Patrick Graham, Bernard Orr)