Hackers broke into UCLA Health System's computer network and may have accessed sensitive information on as many as 4.5 million patients.

UCLA said that it's working with the FBI and had hired computer forensic experts to further secure its network, said the LA Times. 

The university said there was no evidence yet that patient data were taken, but it can't rule out that possibility while the investigation continues, the Times report said. 

Dr. James Atkinson, interim president of the UCLA Hospital System, said the hospital detected unusual activity on one of its computer servers in October and began investigating with help from the FBI.

It wasn't until May 5, according to UCLA, that investigators determined that the hackers had gained access to parts of UCLA Health's computer network where some patient information was stored, said the LA Times. Those parts of the network contained names, dates of birth, Social Security numbers, Medicare and health plan identification numbers as well as some medical information such as patient diagnoses and procedures.

The unauthorized access could have begun in September 2014, UCLA said, and some of the patient information dates to 1990.

Atkinson said it doesn't appear that credit card and other financial information was involved.