A network disruption has been impacting services at hospitals in Downey, Whittier, and Los Angeles.
The disruption means the hospitals cannot make or receive calls, respond to voice or internet messages or review schedules. However, its offices remain open.
Some patients have been unable to get a hold of their doctors and some surgeries had to be canceled.
According to a report published by the Los Angeles Daily News on Thursday, the cyber criminals have hacked the records of 17 million patients that include confidential, personal and medical information.
The hackers sent a demand letter saying, "If you're not going to cooperate and make a deal, then all your confidential files will be published on the internet," according to the report.
The health system says its network has been taken offline as a precaution while forensic specialists investigate the outage.
Patients with scheduled in-person appointments are being asked to arrive early. If any procedures are canceled, patients are being notified directly.
It remains unclear who is responsible for the outage and a potential motive.
Hospital officials released a statement last week, which reads in part: "We are currently working with the assistance of a third-party cyber forensic specialists. At this time, we cannot provide a timeline for when our investigation will be complete."