UCLA's hospital system says information on as many as 4.5 million individuals was stored on servers that were accessed during cyber-attacks beginning last year.
UCLA Health said in a statement Friday that while there's no evidence that hackers acquired personal or medical data, it can't be immediately ruled out.
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Officials say they are working with the FBI to track the source of the attacks. A message seeking further information from the FBI was not immediately returned.
Beginning as early as September 2014, hackers accessed servers containing information including names, addresses, dates of birth and Social Security numbers.
UCLA Health has hired private computer forensic experts to further secure information at its four hospitals and two campuses.
Free identity protection services will be offered to individuals who may have been impacted.