Chinese authorities detained managers from two companies that exported melamine-contaminated wheat gluten and rice protein blamed for the deaths of dogs and cats in the United States.

The deaths set off a U.S. Food and Drug Administration investigation and a recall of nearly 100 brands of pet food made with the tainted ingredients, shedding a harsh light on China's notorious food safety woes and sparking fears that exported products could contaminate food supplies abroad.

China's government body responsible for overseeing food safety said investigators had focused on individuals at two companies blamed for the melamine tainting and said local police had already brought charges. It did not name those detained or give other details.

"Relevant departments will deal strictly with the lawbreaking companies and those responsible according to the results of the investigation," the General Administration of Quality Supervision, Inspection and Quarantine said on its website.

The companies were named as Xuzhou Anying Biologic Technology Development Co. Ltd. and Binzhou Futian Biology Technology Co. Ltd., both listed as having exported melamine-spiked rice protein and wheat gluten to the U.S.

U.S. inspectors said the tainted gluten was used to make pet food and caused the deaths of an unknown number of dogs and cats through liver failure.

Calls to office numbers at the companies rang unanswered on Wednesday. Listed mobile phone numbers for managers at the companies were shut off.

A woman surnamed Cao at the Binzhou police in Shandong province said she could not comment because of the confidential nature of the case.

Police on the provincial, city and county level in Jiangsu province where Anying Biologic is located said they had no information about the case.

The FDA discovered melamine, a chemical used in plastics, fertilizers and flame retardants, in the pet food and traced the contaminated foodstuff to the Chinese companies.

The general administration statement said the companies had added melamine to the wheat gluten and rice protein in a bid to meet the contractual demand for the amount of protein in the products. It said doing so was against regulations.

However, the statement indicated the companies broke the law only when they mislabelled the exported products to avoid inspection.

The general administration said it ordered stepped-up inspections in the wake of its investigation, but that checks on 173 exporters in 11 provinces and cities had turned up no additional signs of melamine tainting after testing of 399 samples.

Additional checks for melamine were ordered on related products, such as processed meats and milk powder, but no contamination was found.

It said those results had been passed on to the FDA, officials from which were in China last week to meet government officials and visit companies. No details of the trip have been released, and it is not known if they are still in China.

"The U.S. side said it hoped for closer co-operation with the Chinese side in the area of boosting animal food safety," the notice said.

The official Xinhua News Agency and Beijing Morning Post newspaper said "related company officials" had been detained, although neither gave further details.

Melamine has no nutritional value but because it is nitrogen rich, it raises the nitrogen level of feed. That makes it appear to be higher in protein and, therefore, garners a better price for the makers of feed for stock animals such as pigs, chickens, and fish, as well as companies that make prepared foods for household pets such as cats and dogs.

Makers of the chemical in China and companies selling animal feed told the Associated Press last week that melamine was widely added to feed. They said they knew of no rules against the practice and were not aware that melamine could be harmful.

Xuzhou Anying managers have said they have no idea how the melamine got into the gluten, which they say was sourced from other firms and sold to a third company that exported it to the U.S.

However, suspicions were raised when the company was found to have posted an advertisement on the website of an online market in March seeking to buy melamine.

U.S. officials say they don't believe melamine to be harmful to humans, but say they have too little data to determine how it reacts with other substances.