Many primary care services remained closed on Friday as security experts worked to determine the extent of the problem and resolve it.

The "data security incident" began Thursday at facilities operated by Prospect Medical Holdings, which is based in California and has hospitals and clinics there and in Texas, Connecticut, Rhode Island and Pennsylvania.

"Upon learning of this, we took our systems offline to protect them and launched an investigation with the help of third-party cybersecurity specialists," the company said in a statement. "While our investigation continues, we are focused on addressing the pressing needs of our patients as we work diligently to return to normal operations as quickly as possible."

Cyber attacks and data hacking have been on the rise globally in recent years. In the US, such attacks have compromised the security of universities, public school systems, hospitals, local governments and federal agencies.

In most ransomware attacks, cybercriminals seek out vulnerabilities in security systems to gain access to data like personal information. They then hold that data hostage, often threatening to make it public or otherwise releasing it unless they are paid a ransom by a certain deadline.