Jeff McIntyre, president of West Virginia American Water, said that officials had set up four labs to test the amount of chemical in the water, but that it might take days to provide enough samples to determine whether the water was safe.

A state official also said that thousands of gallons more of the chemical had leaked into the river than was initially believed…

At a news conference here on Saturday evening, officials said tests had begun to show concentrations of the chemical dropping below the one part per million threshold considered safe by the Centers for Disease Control and Prevention. The concentration must remain that low for 24 hours before the water system can be flushed out and the do-not-use ban can be lifted. Officials said they planned to conduct at least 100 additional tests of samples overnight and on Sunday.

The contamination level was dropping because the leak had been shut off, said Mike Dorsey, the chief of homeland security and emergency response at the State Department of Environmental Protection. Some tests showed concentrations above one part per million, which officials attributed to sludge in the river and rain breaking up contaminated ice along it.