It's a fair bet that most investors would trust the judgment of Warren Buffett, chairman of Berkshire Hathaway, in the running of his companies. Federal prosecutors are apparently not as trusting.

Today, Joseph Brandon, the chief executive of General Re, the reinsurance subsidiary, resigned. His departure comes amid reports that federal prosecutors were pressuring Berkshire to replace Brandon—who had been on a short list of candidates to succeed Buffett—following the convictions last month of four former General Re executives on fraud charges.

"I haven't seen it this explicit," says Peter Henning, a professor at Wayne State University Law School and a founding editor of the White Collar Crime Prof Blog. The last explicit threat he could recall was when Eliot Spitzer, then the New York attorney general, refused to negotiate with insurance broker Marsh & McLennan back in October 2005, forcing the resignation of Jeffrey Greenberg—the son of Maurice Greenberg, formerly of insurance giant American International Group—as chief executive.

"Spitzer went on the warpath. I haven't seen prosecutorial pressure this explicitly since then."

During the trial of the former executives, prosecutors named Brandon an unindicted co-conspirator in the scheme of sham deals between General Re and A.I.G. to prop up A.I.G.'s balance sheet.

After the convictions, Paul Pelletier, one of the prosecutors, said, "We're not done. The investigation continues." He added, "We've got a lot of work to do to work up the corporate ladder."

Did Brandon resign in conjunction with a nonprosecution or deferred prosecution agreement? That is not yet known.

Berkshire did not return calls for comment. Tom Carson, spokesman for the United States attorney in Connecticut, declined to comment on the resignation or on whether the corporation remains under investigation. The trial was a joint prosecution by the Connecticut District, the Eastern District of Virginia, and the fraud section of the Department of Justice, where Pelletier is a lawyer.

It's an ongoing investigation," Laura Sweeney, a spokeswoman at the Justice Department, said of the General Re case. She declined to comment on Brandon's resignation or to confirm news reports that it had been 
prompted by pressure from federal prosecutors.

Henning, for one, does not believe that General Re will be charged. "I think it would have happened a while ago," he said. "Is there a wink and a nod here?"

And while deferred prosecution agreements are "the settlement du jour," he said, the government may have given General Re "a complete pass." (A recent article in the New York Times reported that the Justice Department has entered deferred prosecution agreements with more than 50 companies suspected of wrongdoing during the last three years.)

Another forced resignation of a chief executive occurred in October 2006, when Frederick Lacey, a retired federal judge and former U.S. attorney, served as a monitor of Bristol-Myers Squibb under a 2005 deferred prosecution agreement that followed an accounting scandal. Lacey forced the resignation of Bristol's C.E.O., Peter Dolan, and its general counsel over their handling of a patent dispute.

But white-collar crime defense lawyers say Brandon's resignation does not signal some new trend in prosecutorial overreaching.

"I can't think of more than a handful," says Walter Brown of Orrick, Herrington and Sutcliffe. "A lot of prosecutors will take the position that they are not in the business of telling a company how to run itself."

But the potential for pressure exists, under guidelines for charging corporations released in December 2006 in what is known as the McNulty memo, for its author, Paul McNulty, then the deputy attorney general of the United States. One factor is whether the corporation has taken appropriate "remedial actions," among other things, "to replace responsible management, to discipline or terminate wrongdoers."

That can include firing the chief executive, says Brown. "It is really just the company deciding, based on the smoke signals" sent by the prosecutors, he says. "It's a natural thing to consider."

Like Henning, Brown suspects that General Re is "out of the woods on the allegations."

Brandon will be replaced by General Re's president, Franklin "Tad" Montross.