An industry-run regulator for brokerage firms said there was no record of Mr Madoff's investment fund placing trades through his brokerage operation.
That means Mr Madoff either placed trades through other brokerage firms, a move industry officials consider unlikely, or he was not executing trades at all.
"Our exams showed no evidence of trading on behalf of the investment advisor, no evidence of any customer statements being generated by the broker-dealer," said Herb Perone, spokesman for the Financial Industry Regulatory Authority.
Mr Madoff's broker-dealer operation, Bernard L. Madoff Investment Securities, underwent routine examinations by regulators every two years since it opened in 1960, Mr Perone said.
Mr Madoff, a former chairman of the Nasdaq Stock Market who was a force on Wall Street for nearly 50 years, allegedly confessed to his sons the firm's investment-advisory business was "basically a giant Ponzi scheme" and "one big lie," according to court documents.
He estimated losses of at least $US50 billion from the Ponzi scheme, which uses money from new investors to pay distributions and redemptions to existing investors. Such schemes typically collapse when new funds dry up.
Each month, Mr Madoff sent out elaborate statements of trades conducted by his broker-dealer. Last November, for example, he issued a statement to one investor showing he bought shares of Merck, Microsoft, Exxon Mobil and Amgen among others.
It also showed transactions in Fidelity Investments' Spartan Fund. But Fidelity, the world's biggest mutual fund company, has no record of Mr Madoff or his company making any investments in its funds.
Statement discrepancies
"We are not aware of any investments by Madoff in our funds on behalf of his clients," Fidelity spokeswoman Anne Crowley said.
Neither Mr Madoff nor his firm was a client of Fidelity's Institutional Wealth Services business, their clearing firm National Financial or a financial intermediary client of its institutional services arm, she said.
"Consequently, his firm did not work with our intermediary businesses through which firms invest their clients' money in Fidelity funds," she added.
There also appear to be discrepancies between monthly statements sent to investors and the actual prices at which the stocks traded on Wall Street.
For example, his November statement showed he bought software maker Apple's securities at $US100.78 each on November 12, about a month before his arrest.
But Apple's stock on that day never traded above $US93.24. The statement also showed he bought chip maker Intel at $US14.51 on November 12, but Intel's highest price on that day was $US13.97.
"You could print up any statements you want on the computer and send it out to a client and the chances are the client wouldn't know, because they are getting a statement," said Neil Hackman, president and chief executive of Oak Financial Group, a Stamford, Connecticut-based investment advisory firm.
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