Three former officers of the reinsurance unit of Warren Buffett's Berkshire Hathaway Inc. and one former executive of insurance giant American International Group Inc. pleaded not guilty Thursday to charges stemming from a $500 million accounting fraud.

The four were indicted earlier this month on 13 criminal counts of conspiracy, fraud and making false statements for their alleged role in the book-cooking scheme.

The three former officers of Berkshire's General Re unit appearing in federal court were ex-Chief Executive Officer Ronald Ferguson, ex-Chief Financial Officer Elizabeth Monrad, and ex-Assistant General Counsel Robert Graham.

Also appearing in U.S. District Court for the Eastern District of Virginia was Christian Milton, former head of reinsurance operations at New York-based AIG, the nation's largest insurer.

U.S. District Judge Gerald Bruce Lee set bail at $1 million each and scheduled a May 22 start date for the trial.

The accused and their attorneys declined to comment when approached by reporters outside the courtroom.

The charges stem from a transaction between AIG and General Re in 2000 that investigators said improperly pumped up reserves on AIG's books by $500 million.

AIG has admitted it accounted for the deal improperly and has restated $3.5 billion of earnings over five years and last week agreed to pay $1.64 billion to settle charges with federal and New York state officials.

The matter led last year to the ouster of AIG's long-time chief executive, Maurice "Hank" Greenberg.

In June, two former General Re executives — John Houldsworth and Richard Napier — pleaded guilty to charges that they had helped AIG misstate financial results.

The indictment against the four in court on Thursday referred to a Nov. 14, 2000, telephone conversation in which Monrad allegedly told Houldsworth that the dealings under scrutiny were "being handled at the highest levels in AIG" and that two people at AIG who knew about the dealings were Milton and "AIG unindicted co-conspirator .1."

AIG shares were off 54 cents at $67.75 in early afternoon trading Thursday on the New York Stock Exchange. AIG stock hit a 12-month low of $50 in April last year as reports swirled that further accounting problems had been uncovered.

Berkshire's Class A shares were down $200, or just 0.23 percent lower, at $87,800 per share. Insurance is the biggest business of the investment company run by Buffett.