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Just days earlier, Mr. Madoff had told another senior executive he was struggling to raise cash to cover about $7 billion in requested withdrawals from his clients, and he had appeared “to have been under great stress in the prior weeks,” according to the S.E.C. complaint.

So on Wednesday, the senior executive visited Mr. Madoff’s office, maintained on a separate floor with records kept under lock and key, and asked for an explanation.

Instead, Mr. Madoff invited the two executives to his Manhattan apartment that evening. When they joined him there, he told them that his money-management business was “all just one big lie” and “basically, a giant Ponzi scheme.”

The senior employees understood him to be saying that he had for years been paying returns to certain investors out of the cash received from other investors.

In that conversation, according to the criminal complaint, Mr. Madoff “stated that he was ‘finished,’ that he had ‘absolutely nothing.’ ”

By this account, Mr. Madoff told the executives he intended to surrender to the authorities in about a week but first wanted to distribute approximately $200 million to $300 million to “certain selected employees, family and friends.”

On Thursday morning, however, he was arrested on a single count of securities fraud, which carries a maximum penalty of 20 years in prison and a maximum fine of $5 million.

According to the S.E.C., Mr. Madoff confessed to an F.B.I. agent that there was “no innocent explanation” for his behavior and he expected to go to jail. He had lost money on his trades, he told the agent, and had “paid investors with money that wasn’t there.”

Although not a household name, Mr. Madoff’s firm has played a significant role in the structure of Wall Street for decades, both in traditional stock trading and in the development of newer electronic networks for trading equities and derivatives.

In building those new trading networks, his firm had formed partnerships with some of the largest brokerage businesses on Wall Street, including Goldman Sachs and Merrill Lynch.

Mr. Madoff founded Bernard L. Madoff Investment Securities in 1960 and liked to tell interviewers about earning his initial stake by working as a lifeguard at city beaches and installing underground sprinkler systems.

By the early 1980s, his firm was one of the largest independent trading operations in the securities industry. The company had around $300 million in assets in 2000 at the height of the Internet bubble and ranked among the top trading and securities firms in the nation.

Mr. Madoff ran the business with several family members, including his brother Peter, his nephew Charles, his niece Shana and his sons Mark and Andrew.