Chinese authorities said Monday they have arrested two brothers who ran a private milk-collecting station, on charges of selling toxic and hazardous food, the government-run Xinhua News Agency reported. Police said the elder of the brothers, surname Geng, confessed to putting melamine in milk starting in late 2007.

Melamine is an industrial chemical used to make plastics, glues, fertilizer and other products. It is the same compound that turned up in pet foods sold in the United States and implicated in the illnesses and deaths of thousands of cats and dogs.

Investigators traced the melamine to imports of wheat gluten, a protein source. In February, a federal grand jury indicted two Chinese businesses and their top executives, along with a Las Vegas-based company ChemNutra and its owners, alleging that the Chinese firms had added melamine in an effort to cheaply boost the apparent protein content of the gluten. Melamine, which is high in nitrogen, is not approved for use in food for humans or other animals.

ChemNutra and its owners received the tainted imports in Missouri, then sold the product to pet-food manufacturers, the indictment alleges. The doctored wheat gluten found its way into more than 150 brands of pet food, forcing a massive recall.

The Chinese infant formula became contaminated when the Geng brothers added melamine to their milk after their product had been rejected several times by the dairy company Sanlu Group, police told Xinhua. Sanlu produced the milk powder used in the tainted formula.

The Chinese Health Ministry said Monday that 1,253 infants have been affected, the Associated Press reported. Of those, 53 cases were considered very severe, 340 remain hospitalized, and 913 were only slightly ill. In most cases, the babies developed kidney stones.

Solid masses of crystals that can cause pain and blockage in the kidneys and urinary tract, kidney stones are common, typically showing up in adulthood. However, they also occur in premature infants, according to the National Institutes of Health.

In cats and dogs, the melamine contamination likewise manifested as kidney illness and renal failure.

Dr. Steven Hansen, a veterinary toxicologist and vice president of the American Society for Prevention of Cruelty to Animals, said the dietary practices of infants and pets make them particularly vulnerable: “We have a captive audience eating one specified diet,” he said. “What a tragic situation.”

But Hansen noted that melamine alone apparently is not what poisoned the cats and dogs. Researchers discovered that melamine combined with a second industrial chemical called cyanuric acid caused crystal formation sufficient to shut down kidneys.

“In general, the two compounds alone are not highly toxic,” he said.

Cyanuric acid is a byproduct of melamine production, but contaminated pet foods contained more cyanuric acid than would be expected in a byproduct, Hansen said.

Chinese authorities have not reported the presence of cyanuric acid in the tainted baby formula.

Dr. Paul Pion, co-founder and president of Veterinary Information Network, said the contaminated pet-food and formula cases look suspiciously similar, details aside.

“You’ve always got to hedge your bets, (to) say we’re not sure. But if there’s smoke, there’s probably fire here,” said Pion, a veterinary cardiologist.

Pion said many veterinarians who watched the pet-food scandal closely were concerned the trouble wouldn’t be limited to animals. “The big fear was, if it could happen to dog food, it could happen to human food,” he said.

Last year, the U.S. Food and Drug Administration concluded that the risk was low that melamine contamination would cause a problem in human food, but its research focused on what might happen to people who ate livestock that had been fed the tainted wheat gluten.

With the discovery of poisoned milk in China, the FDA on Friday advised caregivers not to feed babies formula manufactured in that country.

The FDA said no companies that have met the requirements to sell infant formula in the United States are known to have contaminated products. However, the agency said it is investigating whether formula made in China might nevertheless be carried in American specialty markets that serve the Asian community.