Despite the Federal Aviation Administration lifting a nationwide ground stop Friday night, thousands of flights remain grounded Saturday as airlines scramble to resume normal operations.According to the flight tracking site Flig htAware, more than 5,300 flights across the U.S. have been canceled. That comes after a total of 9,147 flights were canceled on Friday.The FAA ended the more than four-hour shutdown of flight departures caused by problems with air traffic control communications.Most major airlines said they will allow customers to change their flights at no extra cost.MORE | FAA nationwide ground stop: What to know about delays, rebooking and refundsTransportation Secretary Sean Duffy said the shutdown was prompted by instability in radar and communications systems at major air traffic control facilities across the country.More than 3,500 flights were airborne when the ground stop was issued, and all landed safely, Duffy said.Saturday morning, he said disruptions are expected to continue and getting people home safely remains the Transportation Department's top priority. He said at a Saturday news conference that he expected airline service to clear its backlog by late Sunday."Operations are slowly returning to normal this morning," Duffy said. "Planes are pushing back, taxiing, and taking off, and it is safe to fly. We are in close coordination with the airlines to support the ongoing recovery. You will continue to see some delays and cancellations as carriers reschedule flights and reposition aircraft/crews where they are needed most."Duffy said Saturday that a faulty router might have caused the communications outage that prompted the Federal Aviation Administration to issue a nationwide ground stop Friday, disrupting thousands of flights and stranding travelers.Investigators have not yet determined the exact cause of the router failure, including whether it stemmed from human error, an installation mistake or a problem with the equipment itself.Homeland Security Secretary Markwayne Mullin emphasized that travelers were not in danger and said officials had found no evidence of a cyberattack."The public was never in danger through this process," Mullin said, explaining that the FAA grounded departing flights out of an abundance of caution."There is nothing that indicates the incident was caused by a cyberattack," he said. "It appears this was mechanically driven."Mullin said the investigation was continuing but reiterated that officials had seen no evidence of a cyberattack. 'This isn't another 9/11': DHS secretary reassures public after ground stopFlight disruptions could continue through SundayAlthough the ground stop has been lifted, Duffy warned that it would take time for airlines to resume normal operations."It's going to take us some time to unwind what's happened," he said.Airlines began announcing cancellations Friday evening as the shutdown stretched on for hours.During the ground stop, domestic aircraft awaiting departure were required to remain on the ground, while flights already in the air were permitted to continue to their destinations.Duffy said he did not believe the disruption would extend into the Thanksgiving travel season.Duffy calls for emergency funding to modernize air traffic controlDuffy pointed to aging technology as a major concern, saying the nation's air traffic control system struggled under the demands placed on it Friday."What we have is an incredibly old air traffic control system," he said.Duffy said communications hardware began losing effectiveness as traffic increased, prompting the FAA to halt departures as a precaution.ON THE GROUND | Passenger forced to rebook flight due to nationwide ground stop: Passenger forced to rebook flight due to nationwide ground stopHe urged Congress to provide additional funding to repair and modernize the system, calling for $50 billion in emergency funding."This is an emergency. I want Congress to give us emergency funding," Duffy said.He warned that similar disruptions could happen again without upgrades."You will have more of these, you will have more of them, unless the American people and Congress tells us to build," he said.The FAA is already undertaking a $12.5 billion overhaul to replace aging physical infrastructure and radio networks with new technology.Air traffic controllers reported communications problemsAn air traffic controller who spoke to Scripps News on condition of anonymity because they were not authorized to speak publicly described how the problems unfolded Friday afternoon.The controller said they received notice of a frequency issue at approximately 3:36 p.m. Eastern, prompting a ground stop at an East Coast airport. Within minutes, two additional control towers reported similar problems. A nationwide ground stop followed shortly afterward.At the time, the controller said personnel had not been told what caused the problems or what repairs were underway."Everyone knows frequencies went down somewhere but no one seems to have actually lost theirs," the controller told Scripps News.The controller also criticized communication from FAA leadership as the situation unfolded."Everyone texting their friends to figure out what's up and no one knows," the controller said.A congressional official separately told Scripps News that neither the White House nor the Department of Defense knew what had caused the disruption at the time.Sen. Maria Cantwell, the ranking Democrat on the Senate Commerce Committee, said the panel planned to review the communications technology used by aircraft and air traffic controllers.Previous nationwide ground stopsThe last nationwide ground stop occurred in January 2023 because of FAA computer malfunctions. Before that, the U.S. halted flights nationwide following the Sept. 11, 2001, terrorist attacks.Friday's shutdown prompted comparisons to 9/11, but Mullin sought to reassure the public that the situations were different."This isn't another 9/11," Mullin said.WATCH: Passengers try to make the best out of ground stop at Las Vegas airport Las Vegas travelers hit the slot machine during nationwide ground stop ...read more read less