A number of top Wall Street investment banks on Tuesday confirmed they were conducting internal investigations into their brokerage businesses, amid fears they may have unwittingly carried out transactions for associates of Osama bin Laden, the prime suspect for last week's US attacks.

The investigations complement an official probe being conducted by US securities investigators.

Officials at the SEC said they were among those searching for evidence that investors used inside information of last week's attacks in New York and Washington to profit in financial markets.

Investigators said there were some transactions that appeared suspicious with hindsight but they had not yet found anything conclusive.

One official was sceptical, however, that criminal groups would risk leaving obvious tracks in the financial markets.

"We're looking at it, but the likelihood you're going to find something is slim," he said. "The kind of money they would make from it just wouldn't be worth the risk."

The banks, too, have expressed doubt of finding anything suspicious.

One top bank, one of the biggest names on Wall Street, which declined to be identified, said: "We, like other banks, have been checking carefully whether there have been any strange-looking transactions. We haven't found any yet.

"We think it's unlikely we'll find anything. We don't do transactions with people we don't know."

US regulators have been in contact with their counterparts in Europe about possible "short selling" of shares.

They have been probing markets amid reports of a series of suspicious transactions completed just before last Tuesday, such as selling short shares of airlines and insurance companies that fell after the attack.

Investigators have been drawing up lists of companies and securities that would have been affected by last week's attacks and then comparing them against indices to look for discrepancies, according to a source.

If it bears fruit, the examination could cast light on the terrorists' financial network.

"If you knew the attacks were coming, you could put up a small amount of money and easily make a 10 or 20-fold return," one hedge fund manager said.