WASHINGTON, Nov 14 (Reuters) - The head of the FAA predicted that US air travel would not return to normal schedules until late Sunday as the aviation system gradually recovers from a nationwide flight shutdown that officials blamed on routine maintenance gone awry.

As of 4:15 pm ET (2115 GMT), airlines on Saturday had canceled more than 5,300 US flights and delayed almost 6,300, 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.

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’ 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 told reporters at a news conference.

L3Harris said it took the issue seriously and is supporting the FAA in its investigation. Its shares last traded down about 4% after hours on Friday.

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, United Airlines and American Airlines all canceled about 25% of their flights on Saturday, while JetBlue canceled nearly 45%. 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.

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 on Saturday told Fox News 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.

(Reporting by David Shepardson in Washington; Editing by Hugh Lawson, Nick Zieminski and Rod Nickel)