The FBI is investigating a threat against water systems in four Midwestern cities, including Wichita, Kan.

FBI spokeswoman Bridget Patton said Friday the FBI hasn’t substantiated any threats to the four cities. She would not name those cities, saying only that they were confined to the Midwest.

Wichita spokesman Van Williams said security in the city’s system is being increased after an unspecified terrorist threat. He says the city’s water is safe to drink.

An internal city email The Wichita Eagle newspaper obtained Friday asked city employees to be vigilant during the next 30 days about anything hooked to a fire hydrant. Another e-mail advised plumbers to call police if they see someone they don’t recognize “messing with a fire hydrant or vent pipe.”

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Associated Press information from: The Wichita Eagle