At least six California residents are suspected of having the mystery illness confounding medical experts around the world. Federal officials on Thursday increased the number of suspected U.S. cases from 11 to 13, and that number is expected to rise.

Severe acute respiratory syndrome, or SARS, has made 264 people around the world ill, according to the World Health Organization. And despite reports from German and Hong Kong labs about possible causes of the disease, U.S. health officials said more tests are needed.

In California, the state Department of Health Services said it reported three new suspected cases to the Centers for Disease Control and Prevention on Wednesday. It earlier reported one Los Angeles case. The new cases are in Placer, Alameda and Santa Clara counties.

All those patients had recently traveled to Asia, where the disease is most prevalent.

In addition, the Los Angeles County Health Department suspects a family member of its earlier case is also ill with SARS. And a sixth case was reported by Colorado when a Santa Clara County resident became ill in Denver.

The CDC has officially acknowledged only three of the California cases so far.

"There's always going to be a lag between what the state and local health officials report and what the CDC reports," said CDC spokeswoman Rhonda Smith.

Several more California cases are under investigation, said Dr. Duc Vugia, the California Health Department's infectious disease chief.

"These numbers change all the time," Vugia said. "It's a very fluid situation."

Still, Vugia and other health officials said they see no evidence that the disease is spreading in California.

What's more, Dr. Laurene Mascola said the disease doesn't appear to be as deadly as initially thought. WHO estimates 14 people have died worldwide, and no U.S. fatality has been reported.

So far, the mystery bug has not been identified as a new flu strain. Instead, health investigators are focusing on a family of viruses called paramyxovirus, a class of bugs that include the measles and mumps.

But Mascola, Vugia and other experts cautioned that it's still too soon to be sure this is the culprit behind the mystery illness. Symptoms of the illness include a high fever, coughing and shortness of breath.