WASHINGTON  Federal drug regulators believe that a contaminant detected in a crucial blood thinner that has caused 81 deaths was added deliberately, something the Food and Drug Administration has only hinted at previously.

“F.D.A.’s working hypothesis is that this was intentional contamination, but this is not yet proven,” Dr. Janet Woodcock, director of the Food and Drug Administration’s drug center, told the House Subcommittee on Oversight and Investigations in written testimony given Tuesday.

A third of the material in some batches of the thinner heparin were contaminants, “and it does strain one’s credulity to suggest that might have been done accidentally,” Dr. Woodcock said.

Two weeks ago, Food and Drug Commissioner Andrew C. von Eschenbach told a Senate subcommittee that the contamination was done “by virtue of economic fraud,” but he quickly withdrew the remark, saying he had “probably gone too far.”