Firefighting foam used in battling a fatal fire in an apartment building entered the public water system, prompting the water district to temporarily order thousands of residents not to drink the water.

The do-not-drink order went into effect Monday for about 9,000 Kennebec Water District customers in Waterville, Winslow, Benton, Fairfield and Vassalboro, and was lifted Tuesday afternoon following testing. People were advised to flush their water lines by running taps for 3 to 5 minutes — or longer if odors or foaming was present.

The foam used by Waterville firefighters is presumed to contain PFAS chemicals, a group of compounds that are widespread, dangerous and expensive to remove from drinking water, but the foam is advertised as being free of fluorine, a toxic element sometimes used in firefighting foam, said fire Capt. Edward Moult.

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"While the extent of the contamination is unclear, out of an abundance of caution, Kennebec Water District is issuing a system-wide Do Not Drink Order," the water district announced Monday.

The foam entered the public water distribution system as firefighters battled a blaze in an apartment building for seniors. One person was killed and several others were injured Monday.

Officials didn't say how the foam entered the public water supply. Newer buildings have a special valve to prevent firefighters' water or foam from flowing back into the public water system, Moult said. The status of such a system on the seven-story apartment building, constructed in 1972, was unclear.

Water samples that were taken Tuesday morning were being delivered to a lab in southern Maine for analysis, and the water district consulted with state health officials before lifting the do-not-drink order.