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.

Members of Congress also are concerned that Chinese heparin manufacturers and their raw-material suppliers didn't fully cooperate with an FDA inspection team in February, after the heparin crisis erupted internationally, and barred the FDA from complete access to some workshops, records and workers.

An FDA compliance official testified to a congressional subcommittee April 29 that the FDA could try to revisit facilities in China, but said, "I cannot say whether they will admit us or not, or whether they will allow us to do a full inspection."

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.