A mysterious intestinal ailment that struck down about 200 people who swam in a Washington lake has pushed officials to shut down access to the lake.

The swimmers who enjoyed a summer weekend at Horseshoe Lake have been reporting symptoms of nausea, cramps and diarrhea, officials said.

The officials suspect a norovirus may be the culprit but are awaiting test results.

In the meantime the popular Horseshoe Lake County Park swimming site has been closed to vacationers until further notice, they say, and warning signs have been posted on the lake shoreline.

“Although there is no laboratory confirmation of a specific virus or bacteria responsible for the illnesses at this time, Public Health and Kitsap County are issuing the temporary closure as a precautionary measure while they investigate and to protect the public and prevent any additional illnesses,” the county public health district said [pdf] in a statement.

The county park is apparently the only area on the lake affected so far, officials said.