U.S. health officials have identified the mysterious substance, which contaminated some batches of the blood thinner heparin associated with 19 deaths and are trying to determine how the chemical got into the drug.

The substance is a man-made chemical compound known as over-sulfated chondroitin sulfate which is much cheaper than heparin. Chondroitin sulfate is a natural compound that occurs widely and is used as a dietary supplement but the over-sulfated version has not been widely studied.

“Right now, people should not be alarmed. We have not received any more reports of fatalities of this type since the recall on Feb. 28. We cannot rule in or out whether this was accidentally or deliberately introduced into the product. We are investigating how it got in,” said Dr. Janet Woodcock, head of the Food and Drug Administration’s Center for Drug Evaluation and Research, the Associated Press reported.

Chondroitin sulfate mimics heparin, so preliminary testing did not identify it, Woodcock said. Oversulfated chondroitin sulfate would be less expensive to make than heparin, but FDA officials said they could not estimate the cost difference.

However, “it is unacceptable that Americans have died and been seriously injured by what appears to be deliberate tampering. 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. 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,” Sen. Edward M. Kennedy (D-Mass.), who chairs a panel that oversees the FDA, said in a statement, according to the AP.

Woodcock said further tests would be made in order to determine “whether or not oversulfated chondroitin sulfate, when combined with heparin, can produce the serious allergic reactions of the sort reported to Baxter and FDA.”

Heparin, made from ingredients extracted from pig intestines, is used to prevent blood clots in millions of people with heart conditions, kidney disease and in surgery. The contaminant made up between 2 and 50 percent of the heparin samples tested by the FDA, which did not assert directly that the heparin was counterfeit on purpose.

Since the end of December, there have been 785 reports of allergic reactions associated with Baxter’s heparin. This compares with fewer than 100 reports of adverse reactions in all of 2007. According to the FDA, there also have been 46 deaths among heparin users.

Following these announcements, Baxter recalled all of its heparin products. The other major heparin supplier, APP Pharmaceuticals Inc., ramped up production to avoid a drug shortage after the Baxter recall. Tests on APP’s heparin have not turned up any contamination.