BEIJING (AP) — Chinese authorities kept concerns about the safety of a Shanghai dairy's products secret for nearly a year before announcing last week that the company had been shut for manufacturing contaminated milk, an official said Thursday.

The delay in notifying the public about the tainted products raises questions about the effectiveness of China's efforts to restore confidence in its food industry after several safety scandals in recent years — including one involving contaminated milk — that exposed serious flaws in monitoring the nation's food supply.

Food safety authorities in Shanghai found contamination in Shanghai Panda Dairy Co. Ltd.'s products last February and started investigations immediately, an official from a district prosecutor's office said. Chinese authorities said the dairy was one of 22 that produced tainted milk in 2008.

They detained three executives in April, but Shanghai's food safety bureau first told the public of the problem only last week when it shut the dairy.

The bureau said the dairy was selling milk powder and condensed milk tainted with the industrial chemical melamine, which can cause kidney stones and kidney failure.

The same chemical had been introduced into infant formula and other milk products in 2008 in one of the country's worst food safety crises. At least six children died and more than 300,000 were sickened after drinking the adulterated milk. The scandal exposed the widespread practice of adding melamine, normally used in the manufacture of plastics and fertilizer, to watered-down milk to fool inspectors testing for protein and increase profits.

"The three executives have been detained since last April but the case, of course, was not allowed to be publicized at the time," said Shen Weiping, a spokesman for the prosecutor's office in Shanghai's Fengxian district, where the dairy company is located. "This is because the case was under investigation."

Food safety authorities have not said if anyone has been sickened by consuming tainted milk products produced by Shanghai Panda. Calls to the company rang unanswered Thursday and its Web site was shut down.

An apparent cover-up by companies involved and safety officials of the 2008 scandal significantly stoked public fury. Many Chinese suspected that because of high-level pressure for the Beijing Olympics to go smoothly that year, some people who were aware of the crisis may have been afraid to speak out.

China enacted a food safety law early last year promising tougher penalties for makers of tainted products that also says authorities should immediately inform the public when food products have been found unsafe for consumption.

Shanghai Panda was one of the dairies named by China's product safety authority in the 2008 scandal. Tests at the time showed its products had among the highest levels of melamine and the company suspended operations amid investigations. It was allowed to resume production after it pledged to improve safety standards.

Last week, however, Shanghai authorities said eight batches of contaminated milk powder and condensed milk produced by the company had been found to contain unacceptably high levels of melamine and would be destroyed.

Yan Fengmin, deputy director of inspection in the General Administration of Quality Supervision, Inspection and Quarantine, told the official China Daily newspaper that both the agency and the Shanghai government were informed immediately after the case was found and that all harmful products were seized.

Shanghai Panda was founded in 2001 with registered capital of 6 million yuan ($880,000) and employed about 60 workers before it closed, according to Shanghai's Oriental Morning Post.

In November, police detained three people suspected of selling tons of melamine-tainted milk powder in northern Shaanxi province — just weeks after China executed a dairy farmer and a milk salesman for involvement in the 2008 scandal.

Copyright 2010 The Associated Press.