RIVERSIDE, Calif. -- The emergency room attendants who fell ill from so-called mystery fumes in February while treating a dying cancer patient probably succumbed to mass hysteria, California's Department of Health Services concluded in a report released yesterday.

It is also plausible, state officials said, that a few hospital staff members were exposed to something that made them ill and that others reacted "to the stressful situation." But if there was an exposure to something, its identity remains unknown, said Dr. Ana Maria Osorio, chief of the health department's Division of Environmental and Occupational Disease Control and co-author of the report.

"There doesn't seem to be a humongous big cloud of something that overwhelmed everybody," she said.

Despite questions still surrounding an unidentified chemical discovered before the autopsy, epidemiology experts said they could find no culprit to explain the bizarre episode that sent six emergency room workers to hospitals after being exposed to patient Gloria Ramirez.

Yesterday's report was the last full-scale study by local and state agencies that have attempted to explain what happened Feb. 19. Each one was inconclusive.

Eleven people reported smelling an unusual odor after blood was drawn from Ms. Ramirez. After five collapsed and the emergency room was evacuated, 23 people altogether complained of at least one symptom -- most commonly headache, dizziness and nausea, the report said.