The regulatory agency now says 69 types of diet pills contain possibly unsafe hidden drugs -- a figure that is probably less than the actual number on the market, FDA new drugs and labeling compliance Director Michael Levy said.

"I think it's fair to say that we have a major initiative investigation ongoing into this type of product," Levy told USA Today.

"We are buying these products, and we are testing them, and we are considering what our options are," he said. "There is definitely the possibility that there could be criminal charges."

The pills using undeclared drugs that may put health at risk include Sibutramine, the active ingredient in the FDA-approved weight-loss drug Meridia, known as Reductil in Europe and most other countries. Sibutramine is a controlled substance that can cause seizures, heart attacks and strokes.

Other undeclared drugs include Rimonabant, the active ingredient in appetite-reducer Acomplia, which failed to win FDA approval and has been withdrawn in Europe; Phenytoin, an anti-seizure medication that has been tied to birth defects and the autoimmune disease Lupus, and phenolphthalein, a laxative that is a suspected carcinogen.

Most of the pills are made in China, an FDA lawyer told USA Today.