armless traces of cyanide were found in the water supply here yesterday, and a continuing rash of anonymous threats prompted officials to turn off taps in four more Louisiana towns.

State police announced the arrest of an adult and three 14-year-olds, bringing to six the number arrested in connection with the threats.

Health officials said they could not tell whether the cyanide found in Hammond water was a naturally occurring trace of the substance or had been introduced deliberately.

Since the first threat Thursday in St. Gabriel, about 55 telephone calls have been received across Louisiana warning of poisoning in public waterworks. Tap water for at least 220,000 residents has been disrupted.

The calls, many presumed to be from "copycats," prompted state officials to urge local officials to keep water flowing unless tests found it to be contaminated.

Hammond Mayor Debbie Pope said cyanide traces at harmless levels were detected in the city's water system after samples were sent to West Payne Laboratories in Baton Rouge. But she said laboratory officials told her "an individual would have to drink about a swimming pool full to kill himself."

Chemist David Strecker, who tested the Hammond sample, said the cyanide could have come from natural sources.