Ever since pet food contaminated with an industrial chemical was traced to shipments of wheat flour from China, American officials have concentrated on cracking down on imports.

It turns out the problem was closer to home, too.

Yesterday, federal officials announced that a manufacturing plant in Ohio was using the same banned substance, melamine, to make binding agents that ended up in feed for farmed fish, shrimp and livestock.

The problem surfaced after a distributor, concerned about what was in its feed binders after the reports from China, sent the product to a private laboratory for testing.

The melamine was used by Tembec BTLSR, a Canadian forest products company with a small chemical plant in Toledo, to make binding agents that keep pellets of animal feed together, said Dr. David Acheson, assistant commissioner for food protection at the Food and Drug Administration.

Melamine is not permitted in food or pet food products. In the last few months, pet food contaminated with melamine, all traced back to China, sickened or killed thousands of pets in the United States.

Dr. Acheson said the levels of melamine and melamine-related compounds in Tembec's products were far lower than that found in wheat flour from China that ended up killing the pets. Consequently, the authorities said that they thought the contamination did not appear to pose a risk to human health.

Nevertheless, the F.D.A. issued a voluntary recall of finished feed made with two binding agents: Aquabond and Aqua-Tec II, which are made by Tembec and used in fish and shrimp feed. A third product, Xtra-Bond, which is made by a Colorado firm called Uniscope using Tembec ingredients, was not recalled because the levels of melamine were low.

Uniscope is the distributor of all three products. Aquabond and Aqua-Tec II are sold mostly overseas; Xtra-Bond is sold domestically.

Agency officials are alerting the foreign governments that received the products, Dr. Acheson said.

The investigation began on May 18 when Uniscope alerted the agency that it had discovered melamine in the testing. Dr. Acheson said that the investigation was in its early stages and that some questions remained unanswered, like how long Tembec had used melamine in its products and the extent of the contamination.

"What Tembec knew, didn't know, what their activities were, is part of the investigation," Dr. Acheson said, at a news conference. Earlier, he said, "It's hard to believe that a manufacturer of pet food would not know about this."

Federal officials would not say whether they would pursue criminal charges in the Tembec case. They said that they had already been contacting domestic manufacturers to make sure they were aware of the sources of their ingredients.

John Valley, Tembec's executive vice president for business development and corporate affairs, said his company thought that Uniscope was shipping the binders overseas for use as shrimp feed. Once the F.D.A. told it that the binders were being used domestically, Tembec stopped making them from melamine, Mr. Valley said.

Tembec makes resins and certain chemicals for industrial uses, including melamine. Mr. Valley said Uniscope was the only customer that used its products for animal feed.

Asked why it was all right to use melamine in feed for shrimp overseas but not in the United States, he said: "Melamine has just really had a focus that's come upon it. A lot of companies and agencies are reviewing certain applications of melamine."

The pet food scandal has led to increased scrutiny of all food imports, particularly those from China, and threatened trade relations. Some members of Congress have demanded tougher inspections of food imports from China.

"This recent incident goes to show that we apparently have some bad actors out there," said Michael Doyle, director of the Center for Food Safety at the University of Georgia. "It can happen in the United States."

But Mr. Doyle pointed out that the incidence of food illness remains relatively rare in the United States. The problem with the Tembec binders surfaced after Uniscope, a company based in Johnstown, Colo., decided to test the binders.

"They just asked themselves, 'I wonder what's in this stuff? I wonder if we have anything in here that shouldn't be in here?' " said Charlie Russell, a company spokesman. "So they sent some samples to a lab."

Tembec has been a supplier to Uniscope since January 2004, Mr. Russell said. Employees at Uniscope, which was founded in 1975 and is family owned, thought that they were buying a resin that was fit for animal consumption, Mr. Russell said.

Uniscope sells the binders to feed manufacturers that mix them with grain and other ingredients to make food pellets for livestock and fish, he said. The binders are sold both domestically and abroad, Mr. Russell said.

According to filings with the Securities and Exchange Commission, Tembec acquired BTLSR Toledo, a custom manufacturer of spray-dry resins, for $8.5 million in 2003.