State and federal teams continue to investigate the chemical spill that shut down the drinking-water supply for 300,000 people in the Charleston, W.Va., area for five days in January.

Starting on Jan. 13, customers of the West Virginia American Water Co. treatment plant, which serves nine counties in and around the capital city, were cleared to use water in certain zones.

"The ban is being lifted in a strict, methodical manner to help ensure the water system is not overwhelmed" by demand, the company said in a statement.

The company had been flushing out the system to remove 4-methylcyclohexane methanol (MCHM), a coal-cleaning solvent. Customers were told to flush out their own systems for 20 minutes after their area was approved for use.

The "do not use" policy was lifted first in downtown Charleston, home to its four hospitals and a heavy concentration of users. There was no timetable announced for the rest of the system.

The go-ahead came after multiple testing by a state, federal and company team showed less than one part per million, or 1 ppm, of MCHM in the water, a level determined to be safe.

A chemical spill on Jan. 9 dumped 7,500 gallons of the solvent into the Elk River, 1.5 miles upstream of the water plant, resulting in a water-use ban for anything but flushing toilets and firefighting.

The state shut down the plant and ordered the contents of the 14 storage tanks—four of them with MCHM—off-site. The state also issued an air-pollution citation to coal-cleaning plant owner Freedom Industries Inc. and mandated a plan of remedial action.

The plant is used only for storage, so its aboveground tanks are not regulated, a Dept. of Environmental Protection spokesman said.

Gov. Earl Ray Tomblin (D), who marshaled health, environmental and public safety workers and got a federal emergency declaration, called the spill "unacceptable."