Federal drug regulators, in announcing Wednesday that the mystery contaminant in heparin was an inexpensive, unapproved ingredient altered to mimic the real thing, moved closer to concluding that Americans might be the latest victims of lethal Chinese drug counterfeiting.

The finding by the Food and Drug Administration culminated a worldwide race to identify the substance discovered early this month in certain batches of heparin, the blood-thinning drug that had been linked to 19 deaths in the United States and hundreds of allergic reactions.

The contaminant, the regulators said, is a chemically altered form of chondroitin sulfate, a dietary supplement made from animal cartilage that is widely used to treat joint pain. The agency’s announcement followed a report Wednesday in The New York Times that was the first publicly to identify the modified substance as the likely contaminant. That report was based on nearly two dozen interviews with researchers and scientists in China, the United States and Canada.

Federal officials stopped short of saying that the contaminant  constituting as much as 50 percent of the active ingredient in heparin  was counterfeit. “At the moment we don’t know definitely whether the contaminant was introduced intentionally or by accident,” said Dr. Janet Woodcock, director of the Food and Drug Administration’s center for drug evaluation and research.