Vandalism that occurred the night before at Midland’s Plymouth Pool has some parents concerned that E. coli might have made their children sick.

City Parks and Services Director Marty McGuire said that at about 5 a.m. that morning, pool employees discovered human feces in the water. The discovery shut down the pool in the morning hours while the city followed procedures for such an event — workers filtered the water and added a ramped-up dose of chlorine.

Later that day, at 3 p.m., five children who went to the reopened pool, each of them under the age of 4, began vomiting. There were stomach cramps, diarrhea and fevers. The illness began within six hours, and each had the same symptoms. When one took her 3-year-old to the doctor, she was told the child might be sick because of E. coli bacteria.

Midland County Health Department Director Michael Krecek, said two water samples at the pool taken earlier this month came back normal and did not detect any contamination, and that the chlorine shock treatment should have wiped out the problem.