The Los Angeles County Board of Supervisors website was hacked Tuesday, with someone substituting a pro-Islamic State group message for links to county information.
The site at www.bos.lacounty.gov was restored Tuesday night.
"No breach of data occurred and no personal information was compromised," a county spokesman said.
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Before county technicians took the site offline, a message was visible in the middle of an all-black screen. Headed by a winged logo with apparently Arabic script labeled Team System DZ, the message carried the headline, "Anti: Govt all word" and went on, "You will be held accountable Trump, you and all your people for every drop of blood flowing in Muslim countries."
After going on to warn that "The Arab rulers do not represent Islam" and issuing additional warnings of accountability, the message ended with "I Love Islamic State."
The message was similar to those that appeared on government sites in Ohio, Maryland, New York and Washington that were hacked Sunday and Monday.
Those incidents are being investigated by the FBI's San Francisco field office, CNN reported.
FBI authorities were not immediately available for comment.
Tony Bell, spokesman for Supervisor Kathryn Barger, said, "Our IT security team will be investigating what happened and offering a corrective action plan to ensure it doesn't happen again."
Bell said he understood the incident wasn't isolated to Los Angeles County and that officials were coordinating with the appropriate law enforcement agencies.