HONG KONG  China’s Health Ministry on Friday announced a nationwide inquiry into the safety of all infant formulas, as a team of investigators from six government agencies descended on the milk powder factory that produced formula now linked to one baby’s death and kidney problems in at least 50 others.

The producer of the suspect formula, the Sanlu Group, recalled 700 tons of it after determining Thursday that it had been contaminated with melamine, an industrial chemical.

The official Xinhua news agency cited Zhao Xinchao, the vice mayor of Shijiazhuang, in Hebei Province, where Sanlu is based, as saying that the police had questioned 78 people, including dairy farmers and milk dealers. The police suspect that some may have diluted milk with water to increase the volume for sale to Sanlu, and then added melamine to disguise the dilution, Mr. Zhao told the news agency.

A common industrial test for the protein content of dairy products or animal feed gives an inaccurately high reading if melamine is present.