The county was one of several government agencies across the country hit in recent days with the same hacking message, including sites in Washington state, New York state, Maryland and Ohio.
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The message on a black background declared "You will be held accountable Trump, you and all your people for every drop of blood flowing in Muslim countries."
The message was from a group calling itself Team System DZ and included several additional messages meant to be warnings.
It is not clear who the group is or whether it is genuinely associated with ISIS.
The message was posted on the county Board of Supervisors website, but not the main county site. The site was taken down in the afternoon while technicians investigated the breach.
County officials say no personal data was compromised from county computers.
The site appeared to be back to its normal operations by Tuesday night.
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"There's no evidence that any personal records of anyone have been compromised at all," said Tony Bell, a spokesman for Supervisor Kathryn Barger.
The attack came on the same day corporate computers across the globe were being crippled by a ransomware virus believed to have been started in the Ukraine.
The virus locks up computer files and then demands a ransom for their release.
Various businesses were shut down across the world as they fell victim to the attack.
The largest terminal at the Port of Los Angeles was also shut down as shipping giant Maersk was taken over by the ransomware.
There is no indication that the Los Angeles County hack is related to the ransomware virus.