A local couple has been held responsible for intentionally tainting milk that killed three children and sickened 36 others in Pingliang, Northwest China's Gansu province, a government official said on Monday.

Wu Zhentu, secretary-general of the city government, said in a press conference that the two, surnamed Wu and Ma, were arrested for allegedly committing the crime.

More than 80 law enforcement officials participated in the investigation, according to Wu.

As of Monday noon, 31 of the 36 injured victims have recovered and been discharged from hospital, while the other five are in stable condition but remain under observation, the secretary-general said, adding the city government will fully cover victims' medical expenses.

According to China Daily, an official from the city government said that before the conference police had had arrested two suspects who confessed to the crime and explained their motivation. The official refused to disclose further details.

Ma Wenxuan, a local dairy farmer in his 40s, who supplied the nitrite-tainted milk to the victims, had earlier been reported arrested.

However, the conference did not reveal that the farmer named Ma was related to the crime.

A head of the local village committee where the incident occurred told the Shanghai Morning Post that Ma had maintained a good reputation in his neighborhood for years.

The report also said police have suspended the operations of two local dairy plants that may be involved in the case.

High concentrations of nitrite were found in samples of the farms' milk and victims' vomit, said Zhu Lemin, deputy chief of a branch of Pingliang public security bureau, on Sunday.

Nitrite is a chemical used to make dyes and to preserve meat, but excessive intake will harm human's health and possibly cause death.

Li Zhuo, one of the victims poisoned by the tainted milk, told media earlier that the milk mixed with nitrite tasted more sour than usual.

Wang Dingmian, former vice-chairman of the Guangdong Provincial Dairy Association, said adding nitrite will not benefit the dairy producers in any way, suggesting the farmers may not be involved.

Lanzhou Morning Post also quoted Zhu as saying that nitrite can be bought at local stores.

However, Li Tao, a doctor from a national-level dairy lab in Harbin, Heilongjiang province, questioned Zhu's explanation and said local officials should uphold their responsibilities.

Li said the finding of intentional crime will downplay the slip of local officials such as market regulators and food-quality control personnel who should also be responsible for the tragedy.

In 2008, at least six children died and some 300,000 fell ill from milk powder laced with melamine, an illegal food additive. Other scandals in recent years have involved clenbuterol pork products, bleached mushrooms and recycled cooking oil.

Kong Qingdong, a professor from the Chinese language and literature department of Peking University, told China Central Television that food safety should start with wakening people's consciousness in eliminating rampant food scandals.