About 6,000 international passengers have been stranded for as long as six hours today on planes and in terminals at Los Angeles International Airport.
Authorities say a computer system used by immigrations and customs officers broke down at about 1:30 this afternoon.
The system contains the names of arriving passengers and law enforcement data about them.
It allows officers to decide who can enter the United States and who can't, Customs and Border Protection spokesman Mike Fleming said.
Terminals that normally accept international passengers were full by 2:30 p.m., and afterward, passengers landing at LAX had to remain on the runway.
Authorities began using a backup computer system by 7:45 and were processing passengers in order of their arrival.
However, Fleming said the system could only support half of the inspection booths normally used.
Officials were also working to divert incoming flights to airports in Ontario, Calif. and Las Vegas.
They did not know why the main system broke down or how long it would take to repair.