After news of the tainted wheat gluten emerged last spring, all of the companies charged in the indictment said they did not know how melamine  which can be toxic when ingested  had made its way into the gluten.

But the owner of Xuzhou Anying, Mao Lijun  who is referred to in the indictment as Mao Linzhun  subsequently conceded to Chinese authorities that his company had added the melamine to make the wheat gluten appear higher in protein, court papers said. Xuzhou was required by ChemNutra to produce wheat gluten with at least 75 percent protein content.

Melamine is used in many industrial products like glues, inks and fertilizers, but it has no approved uses in food in the United States. The F.D.A. found the melamine in the wheat gluten in late March and determined that it had been used in the wheat gluten that Xuzhou Anying shipped from November 2006 to February 2007.

Xuzhou Anying shipped at least 13 loads of the tainted wheat gluten, more than 800 metric tons, through Suzhou Textiles, which created false labels for the shipments, the indictment said.

China’s product safety agency requires that shipments of wheat gluten be inspected before export. To avoid inspection, Suzhou Textiles labeled the shipments with a code other than the one that would have indicated it was wheat gluten, according to the indictment.

ChemNutra and its owners, a married couple, Sally Qing Miller and Stephen S. Miller, knew that the shipment was mislabeled in a way that would allow it to leave China without testing, the United States attorney’s office says, and the Millers did not disclose the mislabeling to pet food makers.

The government cited e-mail messages sent or received by the Millers in 2006 in which the proper codes for wheat gluten and other proteins were discussed.