WASHINGTON — A modified form of a cheap and widely used dietary supplement sold to relieve joint pain was identified as the contaminant found in tainted and at times lethal heparin produced in China for American patients, Food and Drug Administration officials said yesterday.

Agency officials said they were investigating whether the compound was intentionally added to cut costs and boost profits or was mixed in by mistake. In either case, they said it was not part of the prescribed manufacturing process.

“This is a biological compound that is not found in nature,” the director of FDA’s Center for Drug Evaluation and Research, Janet Woodcock, said. “At this point, it’s still to be determined if it was introduced intentionally or by accident.”

The announcement drew an immediate and worried response from members of Congress and drug experts, who said the tainted heparin is a sign of a much larger problem.

“I have no doubt that there are other contaminated or counterfeit drugs like heparin coming into the country, because there’s really no system in place to stop them,” a top official at the FDA until 2005 and part of a consumer and industry group lobbying — so far unsuccessfully — to get more resources for the agency to address the dangers of foreign-made drugs, William Hubbard, said.

A large and growing portion of drugs and drug ingredients for the American market are manufactured abroad, increasingly from lightly regulated nations such as China and India.

Heparin, which has been on the market for six decades, is widely used as a blood thinner in kidney dialysis and surgery.

The contaminant is believed to have caused more than 700 severe allergic reactions in American patients, including a still undetermined number of deaths. FDA officials said the Chinese contaminant was found in lots of heparin associated with the reactions, but it is not certain whether it caused them.

In a statement, Senator Kennedy, chairman of the Senate Health, Education, Labor, and Pensions Committee, said the FDA’s announcement reflects an “unacceptable” situation.

“Whether this contaminant was introduced intentionally or by accident, the full force of the law must be brought to bear to bring those responsible to justice,” Mr. Kennedy said. “To guard against future abuses, every drug manufacturer needs to inform FDA of where it sources its ingredients and what it is doing to ensure that these ingredients are pure and potent.”

Last week, the FDA ordered sophisticated testing of all imported heparin and its active ingredients, either by the manufacturers or the agency.

Ms. Woodcock said the agency feels “doctors and patients now can be confident that the product on the market has been tested and is safe.”