BEIJING - Managers from two Chinese companies which exported contaminated wheat gluten and rice protein blamed for the deaths of dogs and cats in the United States have been detained, Chinese news media reported Wednesday.

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.

The Beijing Morning Post newspaper said China's quality control watchdog had confirmed the Xuzhou Anying Biologic Technology Development Co. Ltd. and Binzhou Futian Biology Technology Co. Ltd. had exported melamine-tainted products and said managers from both companies had been detained.

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 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 two companies illegally 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,'' the General Administration of Quality Supervision, Inspection and Quarantine, was quoted saying in a statement by Xinhua News Agency.

Xinhua said the companies evaded quality checks by labelling the products as export items that are not subject to quality inspection.

It also said "related company officials'' had been detained without giving further details.

Melamine has no nutritional value but because it is nitrogen rich, raises the nitrogen level of feed. That makes it appear to be higher in protein and therefore garnering 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.

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

However, suspicions were raised when the company was found to have posted an advertisement on the website of online market place 999ce.com in March seeking to buy melamine.

U.S. food and drug officials 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 Chinese watchdog body said it had found no further melamine-contaminated products after checking 399 samples from 173 Chinese exporters, Xinhua reported.