Effects of a nationwide ground stop on air travel Friday evening, which stranded thousands of passengers and caused widespread delays, rippled into Saturday as airlines canceled thousands of flights and airports struggled to recover.

The air traffic control system, which faced a faulty upgrade that caused communications failures leading to the ground stop, is expected to be back to normal by Sunday night, said Bryan Bedford, who leads the Federal Aviation Administration. Flight schedules could take several days to return to normal.

Federal officials are still trying to determine who is at fault and have called for congressional funding to prevent similar failures. There is no evidence that the problem was caused by a cyberattack, officials have said.

Here’s what to know:

What caused the ground stop?

L3Harris, a contractor managing the F.A.A.’s telecoms network, made a standard upgrade at a remote air traffic location, federal officials said Saturday. L3Harris was conducting a routine task — replacing copper wire with more modern fiber optic cable — that was expected to pose no significant disruption to air traffic, said Sean Duffy, the transportation secretary.

But the switch from copper to fiber overwhelmed the system with levels of data it could not handle, prompting widespread communications failures across air traffic facilities that manage the national air space. Alarmed at the scope of the problems, officials issued a rare nationwide ground stop for hours.

Administration officials say it’s still unclear whether L3Harris or faulty equipment was to blame. L3Harris said in a statement on Saturday that it was supporting the F.A.A. investigation and that it “takes the nationwide ground stop and its impact on travelers, airlines and air traffic operations seriously.”