Federal officials said the cascading communications failures that forced the government to shut down flights across the United States on Friday started with a contractor upgrading air traffic control equipment, replacing copper wire with fiber optic cable.

The upgrade set off communications failures across the facilities that manage the national air space, the officials said. Alarmed at the scope of the problem, the Federal Aviation Administration decided to issue a rare nationwide ground stop for several hours.

Speaking at news conference in Washington, Transportation Secretary Sean Duffy said that the specific problem with the upgrade, which took place in what he called a “remote” facility, was not yet known. The contractor, L3Harris, has worked for the Federal Aviation Administration for decades, he said.

Duffy said his agency was trying to determine whether the problem had been caused by the contractor’s incompetence or faulty equipment. “We haven’t determined what the fault was,” he said. He added, “We’re going to hold people accountable.”

FAA Administrator Bryan Bedford said there was no evidence that the communication problems had stemmed from a cyberattack or the use of artificial intelligence.

Bedford said that he expected the system to be back to normal by Sunday night. “The worst is behind us,” he said.

But even though the system came back online Friday evening, the effects of the shutdown were vast and continued on Saturday.

Travelers across the United States were still facing delays and cancellations as airlines and airports struggled to recover from the ground stop.

At least 3,000 flights scheduled for Saturday were canceled, according to Cirium, an aviation data firm. At Chicago O’Hare International Airport, a ground stop was issued for United Airlines flights.

Duffy again called on lawmakers to fully fund upgrades that he said were needed to prevent similar failures. To upgrade aging air traffic facilities and technology, he said, he wanted another $50 billion from the Republican-controlled Congress, which last year approved $12.5 billion for the project.

Here’s what else to know:

— If your flight is canceled: You are entitled to a refund if your flight is canceled in the United States, regardless of the reason, or if a flight is significantly delayed or changed and you decide not to take it.

— Rare halt: Friday’s nationwide ground stop was only the third that the FAA has issued this century. It ranks as the second-longest, after the stop issued after the terrorist attacks on Sept. 11, 2001. There was a shorter one on Jan. 11, 2023, because of an outage in the FAA’s safety-alert system.

— Democrats’ response: Rep. Hakeem Jeffries of New York, the House’s top-ranking Democrat, said the stop was “the latest in a series of unacceptable air travel incidents” under the Trump administration. He added that Americans deserved timely answers about how and why it happened.

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This article originally appeared in The New York Times.

By Karoun Demirjian/Niraj Chokshi/Hari Raj/Ryley Ober/Justin L. Stewart

c.2026 The New York Times Company