LAKE ANNA, Va. (AP) — Environmental officials are testing lake water at a popular recreational destination in central Virginia after at least 20 people reported E. coli infections.

At least nine people who sere swimming in Lake Anna have been hospitalized due to the infections, according to the Virginia Department of Health.

Most of the 20 cases are children, the health department said Thursday. An additional 10 cases are still under investigation but E. coli infections have not been confirmed.

The illnesses occurred between May 27 and June 4, according to the health department, a period that coincides with the busy Memorial Day weekend.

The Department of Environmental Quality conducted water testing at the lake on Tuesday, but those results are still pending, according to the health department.

Lake Anna was formed in the 1970s as a reservoir to facilitate cooling at an adjacent nuclear power plant. As a result, the lake famously has a “cold” side and a “warm” side that is heated by water discharged from the plant to assist in cooling.

The health department said the reported exposures have occurred at many parts of the lake.

The health department issued reminders that swimmers in natural waters, like lakes and rivers, should never drink untreated water and should not swim if their skin has cuts or open wounds.

E. coli infection can cause stomach cramps, diarrhea, vomiting and fever. In severe cases, the infection can damage kidneys, and lead to hemolytic uremic syndrome.

Five of the E. coli infections include diagnoses of that syndrome, all of them in children, according to the health department.