Southwest Airlines briefly paused its departures on Tuesday after reporting technical problems, delaying half of its planned flights.

The Federal Aviation Administration said the ground stop had been lifted, but more than 2,100 flights were delayed as of 4 p.m. ET, according to flight-tracking site FlightAware. The delays accounted for more than half of U.S. flights on Tuesday.

“Early this morning, a vendor-supplied firewall went down and connection to some operational data was unexpectedly lost,” Southwest said in a statement. “Southwest Teams worked quickly to minimize flight disruptions.”

Southwest shares fell 0.8% on Tuesday, while the broader market and other large airline stocks rose.

Airlines or the FAA will occasionally pause departures, particularly at certain airports, to avoid bottlenecks for parking on the ground.

Southwest had a meltdown over the year-end holidays, which drove thousands of flight cancellations and resulted in an $800 million pretax hit for the fourth quarter. The airline upgraded scheduling software that couldn’t handle numerous changes to crew assignments during severe winter weather late last year.

The issues Tuesday did not appear to be related to that software.

The previous meltdown increased scrutiny on the airline’s technology from Washington.

“This is another demonstration that Southwest Airlines needs to upgrade their systems and stop the negative impacts to individual travelers,” said Sen. Maria Cantwell, D-Wash., chair of the Senate Commerce Committee, in a statement Tuesday.

Southwest is scheduled to report first-quarter results on April 27 before the market opens.