WASHINGTON, March 5 (UPI) -- Drug counterfeiters may have added an ingredient into Chinese shipments of Baxter's blood-thinner heparin, linked to 19 deaths, a U.S agency said.

A foreign substance similar to heparin's chemical composition was found in as much as 25 percent of the material in the nine suspect lots of the drug, Dr. Janet Woodcock, acting director of the U.S. Food and Drug Administration's center for drug evaluation and research, said Wednesday.

Conventional quality and safety testing performed by Baxter and its supplier, Scientific Protein Laboratories, didn't uncover any variation because the substance was similar to heparin, the Chicago Tribune reported.

"It acts like heparin in this test, so it looks like everything is fine in the test," said Woodcock, who didn't reveal how the material was discovered.

The contaminant was present in the active pharmaceutical ingredient originating from Scientific Protein's plant in China, the FDA said, adding it was possible the substance could have been added at the company's Waunakee, Wis., plant or at Baxter's Cherry Hill, N.J., processing facility.

In a statement, Baxter said the cause "may be associated with the crude heparin, sources from China, or from subsequent processing of that product before it reaches Baxter."