Three children who died after drinking tainted milk appear to have been the victims of deliberate poisoning, Chinese state media says.
Investigators said the industrial salt nitrite had been added to fresh milk at two dairies in the north-western Gansu province in order to harm people.
They had detained suspects in the city of Pingliang, the China Daily reported.
Thirty-six other people, mostly children, were taken to hospital after drinking the tainted milk.
The incident happened on 8 April and as of 10 April 17 people remained in hospital, Xinhua news agency said.
The children that died were all under the age of two, with the youngest only 36 days old.
Food safety scandals in China have badly damaged consumer confidence, particularly in the dairy industry.
Earlier this month, China's quality inspection agency shut down nearly half of the country's 1,176 dairies 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.
The industrial chemical had been added to dairy products to make them seem high in protein.
The scandal led to a worldwide recall of Chinese dairy products and was a major embarrassment for the country's Communist leadership.
Beijing later vowed to constantly monitor milk quality to restore consumer confidence.