A computer system used to usher arriving passengers through customs at airports suffered a nationwide failure for a couple of hours on Friday, causing ripples of delays and leaving frustrated travelers waiting hours to be processed.

The failure was reported around 4:30 p.m., and by about 6:30 p.m., airports said the system had been restored.

It was not immediately clear what had caused the disruption, but the United States Customs and Border Protection said on Twitter that there was no indication that what happened was “malicious in nature.”

Passengers from coast to coast complained on social media about waiting in long lines after extended trips and expressed worry about missing connecting flights. During the shutdown, which affected self-service kiosks, customs officers processed passengers manually, one by one.