The investigations into Bernard L. Madoff are expanding into offshore tax havens.

Federal prosecutors are beginning to consider what role offshore fund operations may have played in the $50 billion Ponzi scheme that Mr. Madoff is accused of orchestrating.

Of particular interest is whether Mr. Madoff and some of his investors used funds based in offshore tax havens to evade American taxes, according to a person briefed on the investigation.

Also under scrutiny is whether certain charities invested with Mr. Madoff had improperly allowed their donors to shift money offshore, and whether foreign banks had withheld American taxes on Madoff accounts, as required by the Internal Revenue Service, according to this person, who was given anonymity because of the delicate nature of the investigation.

Mr. Madoff was arrested Dec. 11 at his New York apartment and charged with securities fraud. The authorities have characterized the fraud as a worldwide Ponzi scheme  perhaps the largest ever.