State media in China say three children who died after drinking tainted milk appear to have been the victims of deliberate poisoning.
Investigators said nitrite, an industrial salt, had been added to fresh milk at two dairies in the north-western Gansu province in order to harm people.
The China Daily reported that suspects had been detained in the city of Pingliang.
Thirty-six other people, mostly children, were taken to hospital after drinking the tainted milk.
Xinhua news agency said the incident happened on 8 April and as of 10 April 17 people remained in hospital.
The children that died were all under the age of two, with the youngest only 36 days old.
The BBC reports food safety scandals in China have badly damaged consumer confidence, particularly in the dairy industry.
Earlier this month, the state quality inspection agency shut down nearly half of the 1176 dairies in the country as part of a campaign to clean up the industry.
In 2008, at least six babies died and another 300,000 were made ill by drinking infant formula tainted with melamine, an industrial chemical.