After discovering an outage in its computerized Notice to Air Missions (NOTAM) systems—which helps guide air traffic and provide crucial instructions to pilots—overnight, the FAA ordered all flights within, into, or out of the U.S. across the country to be grounded until 9 a.m. ET.

But even though flights have resumed, the ongoing congestion at airports across the U.S. is yet to be unclogged. Amid the still-unfolding chaos that the system failure caused, more than 1,300 flights were canceled and more than 10,000 delayed, according to flight-tracking website FlightAware.

Delays are expected to continue through Thursday and possibly longer. Captain Chris Torres, vice president of the Allied Pilots Association, reportedly said that he expects the delay to affect air traffic in the U.S. until Friday.

“This thing was lifted at 9 a.m. Eastern. That doesn’t mean the problem stops at 9 a.m. This is going to cause ripple effects,” he said, as quoted by The Independent.

According to FlightAware, more than 450 flights were delayed as of 4 a.m. ET Thursday and at least 60 canceled.

Southwest, which canceled over 16,000 flights during the holiday period leaving thousands of passengers stranded at a peak time for travel, was among the airlines affected by the system failure, with more than 400 canceled flights on Wednesday. About half of Southwest’s flights were delayed as of 6 p.m. ET Wednesday.

American Airlines reported canceling nearly 400 flights, while other major airlines including United and Delta also reported nearly 40 percent of their flights being delayed or canceled on Wednesday.

Newsweek contacted Southwest, American, United and Delta for comment. This article will be updated with any response.

The cause of the system failure is still unknown, though the FAA said preliminary investigation into the incident “traced the outage to a damaged database file.” The system reportedly stopped working at 8:28 p.m. ET on Tuesday, but the outage was only noticed at daybreak the following day.

The White House, which has directed the Department of Transportation to investigate the outage, said there was no evidence the incident was due to a cyberattack.

Secretary of Transportation Pete Buttigieg, who was harshly criticized online for the incident, said the decision to ground all flights among the FAA system failure was the “right call” to guarantee the safety of passengers, pilots, and crews.