Tampa General Hospital says an “unauthorized third party” hacked its computer network in May and obtained personal data — including Social Security numbers — of about 1.2 million patients.

The hospital says it discovered “unusual activity” on its systems on May 31. An investigation determined the hack by a “criminal group” occurred between May 12 and May 30, according to a statement posted Wednesday on its website.

Tampa General, one of the state’s largest hospitals and a Level 1 trauma center, reported the “cybersecurity event” to the FBI.

“We immediately took steps to contain the activity and began an investigation with the assistance of a third-party forensic firm,” the hospital says. “Fortunately, TGH’s monitoring systems and experienced technology professionals effectively prevented encryption, which would have significantly interrupted the hospital’s ability to provide care for patients.”

A review found the stolen patient data varied by individual. The hospital says its “may” have included names, addresses, phone numbers, dates of birth, Social Security numbers, health insurance information, medical record numbers, account numbers, dates of service and limited treatment information used for business operations.

Tampa General says patients who may have been affected will receive notification by mail.

The hospital says complimentary credit monitoring and identity theft protection will be available to patients whose Social Security number was involved.

“Patients are encouraged to review statements from their health insurer and health care providers, and to contact them immediately if they see any services they did not receive,” the hospital says.

The hospital adds that its electronic medical record system was not involved or accessed.

“The hospital is continuously updating and hardening systems to help prevent events such as this from occurring and has implemented additional defensive tools and increased monitoring,” the statement says.

Tampa General is a private nonprofit hospital with more than 1,000 beds. It is also a teaching hospital affiliated with the University of…