A local Chinese government acknowledged Wednesday that officials knew about melamine-tainted eggs for a month before the contamination was publicly disclosed.
The revelation was the latest in a growing scandal over food products tainted with the industrial chemical.
Contamination of baby formula caused kidney stones and was blamed for killing four infants and sickening 54,000 children. More than 3,600 children remain sick, health officials say.
A brand of chicken eggs from China's leading egg processor, Dalian Hanwei Enterprise Group, was pulled from some stores last week after Hong Kong regulators found excessive levels of melamine.
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Authorities in the eastern city of Hangzhou recalled another company's eggs while Hong Kong's government said tests on eggs from two more processors found excessive amounts.
The government of Dalian, the northeastern port city where Hanwei is based, said in a notice dated Wednesday that it was first alerted to the problem of melamine-tainted eggs more than a month ago – but it did not explain the apparent delay in reporting the problem.
The widening scandal has exposed the inability of Chinese authorities to keep the food production process clean of melamine, despite official pledges to raise standards.
There have not been any reports of people being sickened by contaminated eggs, and it was not immediately clear how many eggs had been recalled.
But the problem was affecting sales. In Beijing, egg sales dropped at least 10 percent Tuesday, the official Xinhua News Agency said.
The Dalian city government's notice said it was likely melamine had been added to the feed given to the chickens that laid the eggs.
Melamine is used in making plastics and fertilizer, and is banned from animal feed. One food industry expert said it was likely added for the same reason cited in the milk scandal and last year's recall of tainted pet food: Melamine boosts nitrogen levels, making products seem higher in protein when tested.