NEW YORK (CNN/Money) - Shares of drugmaker Schering-Plough Corp. fell to a new five-year low Wednesday after the company revealed that its Puerto Rico plant is the target of a preliminary criminal investigation.

In a filing with the Securities and Exchange Commission, Schering (SGP: Research, Estimates) said the U.S. Food and Drug Administration's criminal investigation office is "conducting an investigation which may focus on one or more company products."

The company also said it "has been advised that this investigation is at a very preliminary stage and thus it is not possible to predict its outcome."

Schering has been plagued with regulatory problems at its New Jersey and Puerto Rico manufacturing plants since February 2001. The company's president resigned in June after failing to satisfactorily resolve those concerns.

But in a statement Wednesday afternoon the company said according to the FDA the investigation is not related to previous manufacturing compliance issues.

Schering-Plough spokesman William O'Donnell said all the company definitively knows is what was included in the filing.

Shares of Schering-Plough ended trading Wednesday down more than 12 percent at $25, after hitting a low of $24.26 earlier in the day.

On Tuesday, the FDA sent a warning letter to Pharmacia Corp. about manufacturing violations involving three drugs at its Puerto Rico plant, but no criminal matters were involved.