Two more Southern California hospitals have been attacked by hackers who infiltrated their computer systems with ransomware and demanded payment to unlock the data, officials said.

Chino Valley Medical Center in Chino and Desert Valley Hospital of Victorville, both part of Prime Healthcare Services Inc., had their computer system compromised on Friday by a cyber attack. The cases are now part of an ongoing FBI probe, bureau spokeswoman Laura Eimiller said.

According to sources familiar with the ongoing investigation, the hackers got into one of the hospital’s computers and then spread a malware program that encrypts the data on computers. The hackers then demanded a ransom, typically in a cyber currency, to unlock the servers, according to the sources.

A similar hack occurred this year at Hollywood Presbyterian Hospital, and the hospital paid about $17,000 in bitcoins to get the keys back to its computer servers.

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