PINGLIANG, Gansu - The tainted milk that killed three children and sickened 36 other people in Northwest China's Gansu province was the result of an intentional poisoning, local authorities said.

An investigation team set up by Pingliang city government said in a statement on Sunday that police have nabbed a suspect in the case, declining to give further details.

Police at first were not sure whether the milk that caused the poisoning had been tainted by an intentional crime or simply by improper storage.

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But high concentrations of nitrite - a chemical used to make dyes and to preserve meat - were later found both in tests of vomit samples collected from the victims and of samples of milk from dairy farms, said Zhu Lemin, deputy chief of a branch of Pingliang city's public security bureau.

In China, a thimbleful of nitrite is usually used in meat preservation. The substance can be bought at local hardware stores, Lanzhou Morning Post quoted Zhu as saying.

A Pingliang health bureau official was quoted by media as saying there is no reason to add nitrite to milk. He said the substance even affects the taste of milk.

Police have started an investigation into sales of nitrite, the newspaper said.

An initial inquiry showed that the victims in the scandal fell ill after drinking bulk milk provided by two local dairy farms.

Earlier reports said police detained an independent dairy farmer surnamed Ma, who is believed to have supplied the tainted milk to the victims. It was not known immediately whether Ma is the suspect now being held by police.

Besides Ma, several other people were also being investigated, Zhu said.

Sun Peng, a high school student, said he went to school after drinking raw milk provided by Ma's dairy farm and later "felt a sudden pain in the tummy". His lips, face and even fingernails began to turn dark, he told Lanzhou Morning Post.

In three hours, his grandparents and three other members of his family were taken to a hospital to be treated for food poisoning. They had all drunk milk from the same dairy farm.

The same morning, 39 people were sent to the local hospital after drinking milk. Three children were pronounced dead - all of them younger than two years old; the youngest was only 36 days old.

The Pingliang city government is now trying to ensure that the victims of the poisoning receive proper treatment.

According to the Lanzhou Morning Post, of the 36 victims, one was discharged from the hospital on Friday, and 14 more discharged on Saturday. The other 14 patients are in a stable condition and remain at the hospital.

China Daily - Xinhua