One tweet showed long lines at Houston's Bush International.
On Twitter, people were sending out photos of hundreds of people in line at airports in Miami, Atlanta, New York, Washington D.C. and elsewhere, describing the situation as a "nightmare" and trying to figure out what was happening.
Customs issued a statement saying the agency was experiencing a temporary outage:
"U.S. Customs and Border Protection is experiencing a temporary outage with its processing systems at various air ports of entry and taking immediate action to address the technology disruption," the agency said in a written statement.
"CBP officers continue to process international travelers using alternative procedures until systems are back online. Travelers at some ports of entry are experiencing longer than usual wait times and CBP officers are working to process travelers as quickly as possible while maintaining the highest levels of security."