Residents of Pembina, N.D., a Canadian border town, were warned not to drink the water after a worker found evidence of tampering at the city's water storage tank.
The town's water system was shut down last week and the 500,000-gallon tank was taken out of service. Joe Defoe, a local emergency coordinator, said the lid on the water tank had been tampered with, but it was unclear whether anything had been put in the water.
"You have to play it for the worst-case scenario," said City Council member Warren Hillukka. "This is an indication of our changing lifestyle after Sept. 11."
Francis Schwindt, the state Health Department's environmental chief, said initial tests did not find anything harmful in the water.
But he said chlorine levels in the tank had dropped to zero, which he described as "somewhat unusual."