ANAHEIM, Calif. (AP) — Three new cases of Legionnaires' disease have been identified in Southern California and officials are looking at the possibility there may be a source outside Disneyland, where at least 11 of the patients visited in September, according to reports.

Legionnaires' has sickened a total of 15 people, two of whom have died. The four not connected to Disneyland lived in or had visited Anaheim — suggesting there could be sources of infection not yet discovered, the Los Angeles Times reported Thursday.

The disease can be spread through inhaling droplets from contaminated water sources. While many people have no symptoms, it can cause serious pneumonia and prove dangerous to those with lung or immune system problems.

Disneyland said it learned about the Legionnaires' cases on Oct. 27 and shut down and disinfected two cooling towers where health officials believe the exposure might have occurred. Water samples taken last week from the towers, which are part of air-conditioning systems, tested negative for the bacteria, officials at the theme park said Wednesday.

"Negative results mean that the towers do not pose a current ongoing risk for transmission of Legionella," said Jessica Good, a spokeswoman for the Orange County Health Care Agency. Good told the Orange County Register the agency is working with Disney on procedures to bring the towers back into operation.

Legionella is a bacteria that grows naturally in lakes and streams. When high concentrations grow in man-made water systems, such as air conditioners and plumbing, some people develop pneumonia after breathing in contaminated vapor.

Cases of Legionnaires' disease have been increasing nationwide and in California in recent years, and officials often struggle to identify where the infections originate, the Times said.

Thirteen out of the 15 patients were hospitalized and two, who had additional health issues, died. Neither individual who died visited Disneyland, according to the Register.

Authorities said two of the three new cases involved individuals who visited Disneyland. But officials are also focusing on the four patients who didn't visit the park as they track the source of the outbreak.

"It's too early to point fingers at Disneyland for those four people," said Sanjay Mohanty, a UCLA environmental engineering professor who studies water systems.