The chief executive officer of U.S. drugmaker Baxter International says its blood thinner, heparin, which has been linked to dozens of deaths, appears to have been deliberately contaminated.

Robert Parkinson said Tuesday that a contaminant known as "oversulfated chondroitin sulfate" was found in certain lots of Baxter's Heparin product.

In prepared remarks before a congressional subcommittee, Parkinson also said it was difficult to detect the introduction of the contaminant because of how closely it mimicked heparin.

Eighty one deaths have been associated with allergic reactions to tainted doses of heparin.

The drug is used to help patients undergoing kidney dialysis and other procedures. In addition to Baxter, other companies make the medication. Baxter recalled heparin about two months ago.

Heparin is made from ingredients imported from China. Chinese officials say the material they supply is not the source of the health problems.

Some information for this report was provided by AFP, AP and Reuters.