Citigroup Inc., the biggest American bank, lost $20 million buying and selling gold and silver in 2002 and 2003 after a rogue trader hid contracts and reported bogus prices, New York Stock Exchange documents show.

Gail Edmonds, 40, went as much as 75 times over her trading limit in the month before she was fired in 2003, when Citigroup discovered the hidden positions, according to documents released today. The exchange fined Citigroup $500,000 for inadequate supervision of its precious-metals desk and released findings by a hearing board that investigated the trades.

The case offers another example of lax controls that evolved at New York-based Citigroup under former Chairman and Chief Executive Officer Sanford Weill. Citigroup found in a 2000 internal audit that its precious-metals desk lacked adequate supervision and price verification, and then didn’t do enough to address the failings, the NYSE board found.

The procedures Citigroup adopted “did not provide for reasonable supervision of the price-verification process,” according to the July 17 NYSE hearing board decision released today.”As a result, the firm failed to discover Edmonds’s misconduct for almost one year.”

Citigroup discovered the trades on January 8, 2003, after two customer inquiries about precious-metal prices. By then, Mrs. Edmonds had made trades obliging the bank to deliver $331 million in gold and silver, the NYSE documents said. At one point in December 2002, she had $373 million in open positions.

Charles Prince, who succeeded Mr. Weill as CEO in 2003, imposed tighter controls last year after Citigroup paid more than $5 billion in regulatory and legal settlements. The U.S. Federal Reserve punished Citigroup for its regulatory lapses with a year-long ban on mergers that ended in April, and the bank’s shares have lagged behind peers such as JPMorgan Chase & Co. and Bank of America Corp.

Mrs. Edmonds also lied to a Citigroup clerk about her trading risks, the hearing board determined. She was barred from the securities industry for four years. Mrs. Edmonds and Citigroup didn’t admit or deny the board’s findings.

Attempts to reach Mrs. Edmonds weren’t successful, and her lawyer, Joan Secofsky, wasn’t immediately available.

“We are pleased to have this matter resolved,'”said Citigroup spokeswoman Danielle Romero-Apsilos. “The firm took immediate action when the matter was discovered and we have since strengthened our internal controls.”

According to the NYSE documents, Mrs. Edmonds traded in gold and silver bullion and coins such as American Eagles, Canadian Maple Leafs, Chinese Pandas and South African Krugerrands. In January 2002, she started exceeding her $5 million overnight trading limit by concealing transactions and mispricing gold and silver positions.

By the time she was fired, her trades obliged Citigroup to deliver about 903,300 ounces of gold and 4.3 million ounces of silver.

“As a result of Edmonds’s misconduct, the firm incurred about $20 million in trading losses,”NYSE regulators said in a separate May 30 decision.

Mrs. Edmonds traded on behalf of retail and small institutional customers as well as for Citigroup.The hearing board said her trades on behalf of those customers used “fair and accurate prices” and none suffered “quantifiable” harm.

Citigroup failed to notify customers it overstated the value of some precious-metals trading accounts after discovering the mistake in January 2003, the NYSE hearing board said.The firm instead corrected the accounts in the following month’s statements, according to the NYSE.

Citigroup’s shares rose 39 cents, or 1%, to $49.38 at 1:50 p.m. in New York Stock Exchange composite trading. They have gained 1.8% this year.