The poisonous milk that killed three kids and sickened 36 other people in northwestern China's Gansu Province has been confirmed tainted with nitrite, local authorities told Xinhua.

Tests on the sickened people's vomit samples and samples of milk from the involved dairy farms both showed high concentrations of nitrite, said Zhu Lemin, deputy chief at a branch of Pingliang City's public security bureau.

Several people responsible for the accident were being probed and further investigation into the cause of the accident was under way, Zhu said.

A total of 39 people were sent to local hospitals in Pingliang City Thursday after drinking the poisonous milk. Three children younger than two years old were killed. The youngest was 36 days old.

An initial investigation shows all the patients were poisoned by nitrite after drinking bulk milk provided by two local dairy farms.

As of Saturday, 15 of the patients were discharged from hospital.