A four-hour failure of a processing system used by United States Customs and Border Protection caused delays at airports across the country Monday night, frustrating travelers, many of whom turned to social media to vent about the long lines.

The systems failure, which lasted from about 5 to 9 p.m., did not appear to be caused by a malicious act, the agency said in a statement.

“Travelers at some ports of entry experienced longer than usual wait times as C.B.P. officers processed travelers as quickly as possible while maintaining the highest levels of security,” the agency said.