Intentional poisoning was behind the tainted milk that killed three children and caused 36 others to become ill in China's northwestern Gansu province last week, state media reported Sunday night, adding to the woes of the country's maligned dairy industry.

Local investigators in Pingliang city said they had arrested suspects in connection with the nitrate-laden milk that caused 39 people to seek hospital care last Thursday, Xinhua news agency said. It did not give details on the suspects' identities or motives.

Nitrate, which is used to cure meat, has no use as a milk additive, the official China Daily newspaper cited a health bureau official as saying.

The three children who died after consuming the tainted milk were all under two years old. As of Sunday, 17 of the victims remained hospitalized but were in stable condition.

China's food sector has suffered from frequent poisonings and toxin scandals, and the fast-growing but fragmented dairy sector has been at the heart of those worries.

Earlier this year, Chinese quality authorities sought to calm renewed public alarm after reports that some manufacturers had illegally added a leather protein powder to dairy products to cheat protein-content checks.

In 2008, at least six children died and nearly 300,000 became ill from powdered milk laced with melamine, an industrial chemical added to low quality or diluted milk to fool inspectors by giving misleadingly high readings for protein levels.