Some have reported border patrol officers waiting for the systems to come back online, rather than process people by hand.
CBP added that they were processing passengers manually.
Customs officials haven't figured out what happened, and obviously one thing that comes to mind is a possible cyber attack, although officials haven't said that's even a working theory.
Be glad if you were not traveling Friday afternoon because U.S. Customs and Border Protection (CBP) computers were experiencing a shutdown that caused delays at airports nationwide.
The outage reportedly occurred shortly after 3 p.m. EDT.
The lines at major airports, including JFK, LAX and Dulles, are insane.
"Everyone is well-behaved." said London passenger Jennifer Moline. "People were backing up on the escalator and falling over each other".
LAX Airport tweeted: '@CBP systems are experiencing an issue which appears to be impacting multiple airports including LAX. "Thanks for your patience", JFK Airport tweeted.
Customs officers at Dulles International Airport near Washington haven't been able to process passengers arriving from outside the US since 3:18 p.m. local time, said Micah Lillard, a spokesman for the Metropolitan Washington Airports Authority.