HONG KONG — A month after large batches of Chinese baby formula were found to contain dangerous levels of mercury, state media outlets reported Monday that the authorities have discovered more shipments of contaminated formula, this time poisoned with a cancer-causing toxin.

Health officials in the southern metropolis of Guangzhou said tests of five different milk products showed excessive amounts of aflatoxin, a carcinogenic substance produced by fungus or mold, according to reports by the China Daily newspaper and the official Chinese news agency Xinhua.

The tainted formula, marketed under the Nanshan Bywise brand, was manufactured by two companies in the Hunan AVA Dairy Industry group. Products in another line, Bright Dairy, were still being tested, the newspaper Global Times reported.

Guangzhou officials ordered retailers to stop selling the products while an investigation was begun. There was no verifiable information about the extent of the contamination or whether any children had fallen ill. And it was not immediately known whether Hunan AVA had been told to destroy, recall or do further testing on its products.

Last month, China’s largest dairy firm, Inner Mongolia Yili Industrial Group, found elevated levels of mercury in its infant formula and was forced to recall six months’ worth of production. Yili was one of the dairies involved in a 2008 milk scandal over melamine poisoning.

Chinese dairy executives say independent, small-scale farmers who bring their milk to producers are the likely culprits in contaminations.

“Most people don’t understand that dairy farming requires highest scientific approach to the operation,” said Charles Shao, the CEO of Huaxia Dairy Farm, quoted by Xinhua. “If you compare with the American farmers, the average Chinese dairy farmers are at least 50 years behind.”

Safety concerns about milk, milk powder and infant formula have become a highly charged topic in China in recent years, an issue so volatile that Internet searches about the continuing episodes of contamination are sometimes blocked by government censors.

Rendezvous recently explored the food-safety mess in China, where baby formula is just one of the hot-button issues. Imitation soy sauce has been made from hair clippings, ink and paraffin are used to dress up cheap noodles, and pork buns get so loaded with bacteria that they glow in the dark. There’s formaldehyde on the cabbages. There’s chlorine in the soft drinks. There’s chromium in the gelatin capsules.

Things have gotten so bad that a new iPhone app has been launched to track food scandals nationwide. The app, which sends out daily updates on the latest outrages, was reportedly downloaded more than 200,000 times in the first week.

There was a major aflatoxin scare last December when a large milk producer, China Mengniu Dairy, had to destroy large batches of milk. Within a day of the news, as The New York Times reported, people on the Internet “had posted or copied posts on the bad milk nearly four million times.”

The company apologized for the contamination but also blamed dairy farmers for poor quality controls on their cows and feed.

A Mengniu spokesperson, Lu Jianjun, was quoted on Hexun Weibo as saying “the reason the milk is tainted is because the feed used by dairy farmers was moldy, but a dairy company can’t control the feed used by dairy farmers.”

“Just like a country legislating that one can’t commit murder but some people simply want to kill people, so what can you do?” Mr. Lu said, according to the Hexun Weibo post, which was shown on the ChinaSmack blog. “There is not a single manufacturer whose feed won’t become moldy. This is just an isolated incident, just like how a steamed bun in your home can also become moldy.”

The tenor of the company’s response enraged many netizens in China, and hackers blackened the home page of the company’s Web site, ChinaSmack reported. The hacking team left a message that said, in part:

“So this is what Mengniu Dairy Company is like. As a Chinese person, you should wake up, and our duty, the duty of millions of netizens, is to make you wake up. To wake China up. This is our nation’s shame. “Mengniu once strengthened the Chinese people, once made Chinese people proud. China’s own dairy company, now screws over its own people. Are you a cheat, a cheat, or a cheat? One half kilogram of milk each day, makes Chinese people dead. [This is a play on a Mengniu slogan: One half kilogram of milk each day, makes Chinese people strong.]”

Wang Dingmian, former vice chairman of the Guangdong Provincial Dairy Association, told Global Times on Monday that aflatoxin is typically formed when fodder such as peanuts and corn are stored without proper ventilation.

In the 2008 melamine scandal, various milk products, notably baby formula and milk powder, were found to contain melamine, an industrial additive used to make fertilizer and industrial piping. Dairy producers were using the melamine to make their milk appear to have higher protein levels than it actually did.

Six children died in the scandal and some 300,000 fell sick amid a nationwide panic.

There was a worldwide recall of Chinese products ranging from biscuits to baby formula. The scandal brought down one of China’s largest dairy firms, and two milk producers were executed for selling more than 3 million pounds of contaminated milk powder.