Police in northern China have detained 22 people after smashing a network that made and sold the industrial chemical melamine and added it to milk, state media said on Monday.
Nineteen of those detained in Hebei province were managers of "pastures, breeding farms and milk purchasing stations," Xinhua news agency reported.
The detentions came after more than 800 police raided 41 sites in the city of Shijiazhuang, seizing 222.5 kilogrammes of melamine.
"According to police investigation, melamine was produced in underground plants and then sold to breeding farms and purchasing stations," Xinhua said.
Shijiazhuang is the headquarters of Sanlu Group, the company at the centre of a tainted dairy product scandal in which 53,000 children have fallen ill and four have died.
Formal arrest warrants have been issued for 13 of those detained in the raids, Xinhua said.
The agency had previously reported 18 detained. Its report on Monday did not specify whether that earlier figure was included in the 22.
China has vowed to bring to justice those responsible for the scandal, which has further tarnished the image of Chinese goods following a series of product safety scandals in recent years.
Melamine is a chemical normally used in making plastics and fertilisers. It is believed to have been added to milk supplies to give the appearance of higher protein content.
Consumption of melamine can lead to the development of kidney stones and other urinary ailments.