US air travel is not expected to return to normal ​schedules until late on Sunday, the head of the FAA said, as the aviation system gradually recovers from a nationwide flight shutdown that officials blamed on routine maintenance ‌gone awry.

As of 9:35 pm ET (0235 GMT), airlines on Saturday had canceled more than 6,400 US flights and delayed more than 8,000, according to FlightAware, an aviation website. That followed about 9,200 flights canceled and 7,500 delayed on Friday after the FAA ordered a nationwide ground stop because of disruptions to air traffic communications. FlightAware said 875 flights scheduled for Sunday were already canceled.

Federal Aviation Administration head Bryan Bedford said on Saturday that he expected airline operations to largely ​return to normal by late Sunday. He also apologized for the disruption, adding that the system remains stable.

The FAA issued a nationwide ground stop on Friday that halted nearly all departing ​flights for almost five hours. It was the longest nationwide halt to air travel since the attacks of September 11, 2001, and one of the ⁠single worst US travel days ever.

The issue prompted the White House to convene an emergency meeting in the Situation Room on Friday and federal agencies to scramble to determine if it was a ​cyberattack. But the problem was traced to FAA contractor L3Harris’ (LHX.N) installation of at least one new router at a remote facility, Transportation Secretary Sean Duffy said on Saturday.

“We’re working to determine whether we had incompetence, ​meaning was it user error or installer error from L3Harris, or did they install a faulty router? We haven’t determined what the fault was,” Duffy said at a press conference.

The Professional Aviation Safety Specialists, a union representing FAA safety employees, said the issue was isolated to two routers that were updated Friday. An official told Reuters it was believed the routers were in Arkansas and Texas.

An FAA official said the agency conducted a safety assessment before the work. ​The union said PASS-represented technicians were not involved in the upgrades that led to the ground stop.

Harris said it took the issue seriously and is supporting the FAA in its investigation. Its shares last ​traded down about 4 percent after hours on Friday.

Overloaded Communications Centres

Duffy said the trouble began on Friday afternoon, when air traffic control centers experienced instability in radar and communications systems. The router issue led to data overloading other ‌communications centers, ⁠officials said.

“We had over 1,000 connection attempts every four to five seconds, and what that did was (it) gave us issues with our communication with airplanes, and it also caused some disruption with our radar,“ Duffy added.

The issue affected communications at 17 of the FAA’s 22 en route centers, which control aircraft during the cruise portion of a flight. Bedford said the FAA ordered the ground stop in case a center lost all communications. “No facilities went dark. Nothing was fully lost,“ Bedford added.

The FAA said it was doing additional testing with L3Harris to see if there are any additional risks.

Delta Air Lines (DAL.N), United Airlines (UAL.O), and ​American Airlines (AAL.O) all canceled about 25 percent of their ​flights on Saturday, while JetBlue (JBLU.O), canceled nearly 45 percent. ⁠Airlines have issued travel waivers allowing passengers to rebook flights for later dates without cost.

Though the FAA lifted the ground stop on Friday evening, the disruption for passengers was far from over, with tens of thousands of people stranded in airports. Airlines were trying to get flight crews and other personnel in ​position to resume flights.

The disruption stranded some college football teams on Friday before games scheduled for Saturday.

Funds Sought to Fix System Strain

Friday’s disruption is ​the latest and most dramatic sign ⁠of strain on a US air traffic control system beset by aging technology, equipment and communications problems and a shortage of several thousand controllers.

Duffy called on Friday for $50 billion in funding to upgrade air traffic control systems, up from his earlier request of $30 billion — after Congress approved $12.5 billion last year.

House Speaker Mike Johnson told Fox News on Saturday that he supports approving more funding for air traffic this year. Johnson’s Republicans control Congress ⁠until next year, ​when Democrats will take over because of their gains in the November 3 midterm elections.

US Representatives Rick Larsen and Andre ​Carson, Democrats slated next year to chair the House Transportation and Infrastructure Committee and aviation subcommittee, respectively, said in a joint statement Saturday that their panels will “hold the administration accountable for what happened to make sure this chaos does not happen ​again.”

Since receiving $12.5 billion last year, the Transportation Department has awarded contracts worth $4.3 billion but only spent $1 billion, Larsen and Carson said.