Police in north China's Hebei province have detained 22 people involved in a network connected with making and selling melamine, and tainting milk with the chemical.
Among the detained, 19 were managers at 17 pastures, breeding farms and milk purchasing stations. The local procuratorate has issued arrest warrants for 13 of them, according to the investigation panel of the Sanlu Group tainted milk case.
More than 800 police officers conducted raids on 41 pastures, farms and milk purchasing stations in Shijiazhuang, the provincial capital, and seized 222.5 kg of melamine.
According to the police investigation, melamine was produced in underground plants and then sold to breeding farms and purchasing stations.
One of the suspects, surnamed Gao, said he had been producing "protein powder" containing melamine in Dingxian county since late last year. A man surnamed Xue helped him sell the powder to purchasing stations.
In another development, a month-long overhaul of the country's milk collection system was launched recently around the country to guarantee the safety of dairy products.
The Ministry of Agriculture has sent eight supervision teams to milk collection centers, dairy farms and feed enterprises in 16 provinces and regions to examine their operations, a ministry spokesman said on Sunday.
Meanwhile, 31 provinces have set up local special organizations headed by vice-governors to implement the overhaul.
It is expected at the end of the overhaul the feeding of cows and milk collection could be standardized, the spokesman said.
The milk collection system has for the tainted milk scandal that has killed four babies so far.
Minister of Agriculture Sun Zhengcai said earlier that the milk collection centers began operating only in recent years, and the government does not have a specific department to supervise them.
The milk collection centers have gone "out of control", and melamine was most probably mixed with milk there, where small dairy farmers sell their products, he said, promising to clean up the system.
So far, 26 provinces have come up with detailed overhaul plans.
Local governments in the Inner Mongolia autonomous region, Henan, the Ningxia Hui autonomous region, Guangdong and Jiangsu have started to register every milk collection center and send investigation teams to them.
It will help prevent contaminated milk from reaching food-processing plants, the spokesman said.
Xinhua contributed to the story