More than two dozen people have been diagnosed with gastrointestinal illnesses potentially linked to raw oysters consumed at four Los Angeles County restaurants, health officials said Friday.

County Department of Public Health officials said they are working with state health officials to investigate and verify the source of the cluster of illnesses, which has so far involved 27 associated cases.

The names and locations of the restaurants were not released.

“Until the source is confirmed, consumers should be cautious before eating raw oysters due to the potential risk of foodborne illness,” Dr. Muntu Davis, Los Angeles County Health Officer, said in a statement. “If you are sick, avoid spreading illness by washing your hands frequently and cleaning frequently touched surfaces such as doorknobs, light switches, and kitchen countertops.”

Health officials said more vulnerable residents, such as young children, older adults and those who are immunocompromised, should be aware of the risks of consuming raw oysters.