The message read, "Emergency Leave the Terminal," the Los Angeles Times reported, and was displayed for about 5 minutes in one of the terminals at around 10 p.m.
Travelers noticed the sign and quickly alerted airport officials, the Times said.
Law enforcement officers began searching for who was responsible for the breach and reviewed computer records to determine who may have hacked the system.
But airport officials said at 2 a.m. Tuesday the display of the message was triggered by accident by an airline contract worker with authorized access to the system.
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