Ms Woodcock said the FDA believed the contamination was deliberate
America's drugs watchdog believes that Chinese-made ingredients for a blood-thinning drug may have been deliberately contaminated.
The US Food and Drug Administration (FDA) said earlier that a chemical contaminant had been found in some batches of the drug heparin.
It has linked the contaminant to hundreds of severe allergic reactions and dozens of deaths across the US.
An FDA director said when and how the toxin had been introduced was unclear.
"FDA's working hypothesis is that this was intentional contamination, but this is not yet proven," Director of the FDA's Center for Drug Evaluation and Research Janet Woodcock told a congressional hearing.
Heparin is used in kidney dialysis and in some types of surgery to prevent blood clots.
The contaminated stocks were made by Baxter International, using China-based suppliers.
According to the FDA, a chemical called oversulphated chondroitin sulphate has been found in supplies in 13 countries.
Baxter International Chief Executive Robert Parkinson said the company was "alarmed that one of our products was used in what appears to have been a deliberate scheme to adulterate a life-saving medication".
The company has recalled the heparin and imports from the Chinese supplier have been blocked.
Chinese officials have acknowledged the presence of the contaminant.
But they say it is not to blame for the allergic reactions or deaths, citing a lack of incidents in more than 10 other countries which have received the same contaminated stocks.