Three people died and five were hospitalized after eating Thanksgiving dinner at an American Legion Hall in Antioch, California, an official told the Los Angeles Times.
Dr. Louise McNitt, the director of infectious diseases for Contra Costa County Health Department, told the LA Times that the attendees were elderly and all lived in the same residential facility.
Health officials are trying to determine what caused the tragedy.
Marilyn Underwood, director of environmental health for the county health services, told the Los Angeles Times there is no reason to conclude the food at the Thanksgiving event made the residents sick.
Their symptoms included nausea, diarrhea and vomiting.
The eight affected, none of whom have been identified, were among more than 800 who attended the Thanksgiving meal.
The event, which was hosted by Golden Hills Community Church, was open to members of the public and welcomed families, homeless people and residents of adult care facilities.
"They fed anybody that wanted to come," Underwood told the Los Angeles Times. "It's all sort of folks that come to the event."
Officials told the Los Angeles Times they don't think the incident was an outbreak of food-borne illness because the affected people lived in the same assisted living facility.
McNitt asked people to come forward if they attended the Thanksgiving dinner and felt sick.
"We're interviewing people that we know are ill and also trying to find people that may have had some symptoms but did not seek any healthcare," McNitt told the Los Angeles Times.
Health officials added that the incident is not a threat to the general public.