WASHINGTON -- The Food and Drug Administration is withholding a list of Chinese heparin suppliers requested by congressional investigators looking into problems with tainted supplies of the blood thinner, saying confidentiality agreements prevent release of the companies' names.

Tensions between congressional Democrats and the FDA over the agency's handling of the heparin problem and its willingness to disclose information have escalated in recent weeks.

"The FDA thinks they have it under control, but they really don't," said the congressman leading the investigation, Rep. Bart Stupak (D., Mich.). The FDA's reluctance to release the Chinese companies' names is a red flag, he said. "If I was the FDA director, I'd shut down every drug coming in from China" until they were deemed safe, he said.