A South Florida city warns its residents about a scam. It starts with water testing.

The city of Homestead warned residents and businesses Thursday: if anyone comes to your door saying they’re from a utility and want to test your water, call the city or the police. Do not let them inside.

Fake water testing starts a fraud designed to sell a pricey water filtration device, the city cautioned.

The scheme, as described in Thursday afternoon’s alert, is fairly straightforward:

A person saying they’re from a utility or Homestead Public Services says they’re testing water quality in local businesses and homes. After faux water testing, the person says the water is “severely contaminated.” The problem can be cleared up with a water filtration device they have for sale.

“Please be advised that the City of Homestead and Homestead Public Services are not conducting door-to-door water testing and there is no current issue with water quality,” the city said.

Residents who get visitors such as this are asked to call Homestead Customer Service at (305) 224-4800 or Homestead Police at 305-247-1535. Scams also can be reported to the Florida Department of Agriculture and Consumer Affairs, 800-435-7352 (HELP-FLA); the Financial Crimes Enforcement Network, 800-767-2825; or the Federal Trade Commission, 877-382-4357 (FTC-HELP).

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