A chronology of events in the tainted baby formula scandal that has been blamed for the deaths of four infants and for sickening nearly 53,000 others in China:

_ December 2007: Sanlu Group Co. receives complaints from consumers that its infant formula was sickening babies, according to state broadcaster China Central Television, citing findings from a State Council investigation.

_ June 2008: Sanlu learns its milk powder is contaminated with melamine, CCTV says.

_ June 30: China's product quality watchdog receives a complaint that five infants at the Hunan province children's hospital have kidney stones and were all fed Sanlu brand milk powder, according to data on the Web site of China's food safety watchdog that has since been removed.

_ July 24: A pediatrician tells the watchdog he has seen nine cases of kidney stones in infants, all of whom drank Sanlu formula. The unidentified doctor expresses concern over the milk powder, according to data on the watchdog Web site that has since been removed.

_ Aug. 2: Sanlu alerts the government of Shijiazhuang, the northern Chinese city where the company is based, that its infant formula is tainted, according to CCTV. At a Sanlu board meeting, New Zealand dairy farmers' group Fonterra, a major investor in the company, learns about complaints of sick babies and urges an immediate recall.

_ Aug. 6: Sanlu pulls baby milk powder from distributors, but does not do a public recall.

_ Aug. 8: The Beijing Olympics open and run until Aug. 24.

_ Sept. 5: Fonterra informs New Zealand's Prime Minister Helen Clark about the issue. Three days later, Clark orders New Zealand officials to inform officials in Beijing.

_ Sept. 9: Shijiazhuang city officials inform Hebei provincial officials. A day later, Hebei provincial officials contact the central government.

_ Sept. 11: Sanlu publicly recalls 700 tons of baby formula. Government vows "serious punishment" for those responsible. Xinhua reports dozens of babies have kidney stones and one baby has died.

_ Sept. 13: Vice Health Minister Gao Qiang says 432 babies fed Sanlu formula have developed kidney stones. Gao blames Sanlu for delays in warning the public and launches an inspection of all Chinese formula companies. Hebei's vice governor says authorities have seized 2,176 tons of Sanlu formula and recalled 8,218 tons.

_ Sept. 15: Number of sickened children rises to more than 1,200, with two deaths. China's quality watchdog says contamination occurred mainly at farms, which sell raw milk to dairy companies. Sanlu's vice president apologizes, but does not explain the delay in informing the public.

_ Sept. 16: A nationwide inspection of 109 Chinese baby milk powder companies uncovers 22 with traces of melamine. Sanlu General Manager Tian Wenhua is fired and dismissed from board of directors, state media reports.

_ Sept. 17: China's two largest dairies, Mengniu Dairy Co. and Yili Industrial Group Co., recall baby formula. Health minister says three babies have died and more than 6,200 others have been sickened. China deploys 5,000 inspectors to companies producing baby milk.

_ Sept. 18: Authorities arrest 12 more people, bringing total detained to 18. Police confiscate 660 pounds of suspected chemicals.

_ Sept. 19: Crisis widens after government tests find melamine in liquid milk produced by three of China's leading dairies.

_ Sept. 21: Health ministry reports the number of sickened children soars to nearly 53,000, with 12,892 hospitalized and 104 in serious condition. The Chinese territory of Hong Kong reports the first known illness outside mainland China, a 3-year-old girl who developed kidney stones after drinking Chinese dairy products.

_ Sept. 22: The head of China's product quality watchdog agency resigns.