The cascading collapse of U.S. air travel on Wednesday caused by an air-safety system’s failure was attributed to human error, a senior official with knowledge of the matter told ABC News. An engineer conducting routine maintenance “replaced one file with another,” resulting in the breakdown, the official said. The cause of the system’s disintegration was not immediately apparent to Federal Aviation Administration staff, including the engineer in question. “It was an honest mistake that cost the country millions,” the official said. Reuters and Bloomberg News reported that FAA officials believed the outage to be tied to a corrupt digital file, which affected not only the main system but also its backup. A roughly 90-minute ground stop was issued by the agency after doubts were raised about the backup system’s functionality, Transportation Secretary Pete Buttigieg explained to reporters earlier on Wednesday. The ABC News report did not identify whether the file the engineer had moved was the corrupted one pinpointed by agency officials.