WASHINGTON, April 29 (UPI) -- The U.S. Federal Drug Administration said Tuesday it suspects contamination of the blood thinner heparin that led to 81 deaths was deliberate.

"FDA's working hypothesis is that this was intentional contamination, but this is not yet proven," Dr. Janet Woodcock, director of the agency's drug center, told the House Subcommittee on Oversight and Investigations in written testimony.

Woodcock went on to say "it does strain one's credulity to suggest" that one third of the material in some batches of heparin could have been contaminated accidentally, The New York Times (NYSE:NYT) reported.

The disclosure of the FDA's current theory of what happened comes two weeks after FDA Commissioner Andrew C. Von Eschenbach told a Senate panel the contamination occurred "by virtue of economic fraud" only to withdrew the comment as probably going "too far."

Scientific Protein Laboratories Chief Executive David G. Strunce, whose firm supplied contaminated heparin material to Baxter International (NYSE:AX), which produced and marketed the finished drug, labeled the contamination "an insidious act."

The FDA. has named a Chinese company, Changzhou SPL, a subsidiary of Scientific Protein Laboratories, as the source of the contaminated heparin.