WASHINGTON, Aug. 17 (UPI) -- Medical investigators say an Illinois woman who died while in an experimental gene therapy trial was infected with a fungus that spun out of control.

No formal cause of death has been determined for Jolee Mohr, 36, who died July 24.

Doctors said she had been generally healthy until July 2, when genetically engineered viruses were injected into her right knee in an experimental treatment for her rheumatoid arthritis, The Washington Post said Friday.

A doctor involved in the investigation said Mohr was infected with histoplasmosis, a fungus common in the area in which she lived. It had ravaged her organs, suggesting that her immune system was seriously impaired.

At the time of her death, the Taylorville, Ill., woman was taking conventional immune-suppressing drugs for her arthritis. One of the drugs, Humira, is known to make patients susceptible to histoplasmosis, the newspaper said.

"It's a major mystery," said Kyle Hogarth of the University of Chicago Medical Center.

Targeted Genetics of Seattle said none of the more than 100 other volunteers who got the injections suffered anything more than short-lived side effects.