(CNN) The strain of E. coli causing the current outbreak in romaine lettuce has been found in a reservoir on a farm in Santa Barbara County, California, the US Food and Drug Administration and the Centers for Disease Control and Prevention announced Thursday.

The agencies are continuing to investigate other possible sources, and the CDC still advises consumers not to eat romaine lettuce grown in California's Monterey, San Benito and Santa Barbara counties until investigations are complete.

"We cannot say how many cases are linked to this specific farm at this time," said Ian Williams, chief of the CDC's outbreak response and prevention branch. "We have to do additional work at this farm and other farms that are being identified from our investigation."

Properly labeled romaine grown outside those three counties and harvested after November 23, as well as romaine grown in greenhouses or hydroponically, should all be safe from contamination, the CDC said. The earlier warning against eating romaine from California's San Luis Obispo, Santa Cruz and Ventura counties has been lifted.

The agency stressed that consumers should continue to avoid any romaine lettuce not labeled with the harvest date and location.

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