America's top market regulator announced several US agencies were investigating whether those behind last week's terrorist attacks tried to turn a profit by playing the markets.
"There is a multi-agency effort under way, under the leadership of the Federal Bureau of Investigation, but my agency is actively investigating reports of possible terrorist activity," said US Securities and Exchange Commission (SEC) chairman Mr Harvey Pitt at a US Senate Banking Committee hearing.
US Treasury Secretary Mr Paul O'Neill, who also testified at the Senate hearing, said his department was working with the SEC. He said it was unclear whether there was unusual trading in airline stock options prior to the attacks.
As the focus of the global markets investigation shifted to Washington, regulators in France and elsewhere also said they were on the hunt for suspicious trading.
France's stock market watchdog said it opened a formal inquiry into trading in the days leading up to the September 11th airliner attacks that left more than 6,500 dead or missing.
US and French efforts resemble inquiries in Chicago, Germany and Japan into whether anyone aware of the attack planning tried to profit from that inside knowledge by selling short or buying put options related to stocks that have since lost value, including airline, insurance and banking issues.
Selling short and buying puts are common, but highly risky, ways of making money on falling share prices.
A put option is a contract to sell an underlying security at a fixed price by a specified date. A call is a contract to buy under similar terms. Both are traded on the Chicago Board Options Exchange, the world's largest options market which said it is looking into pre-attack market activity.
Short-selling is another common, but risky way to exploit a falling share price. Short sellers borrow shares and sell them, then repurchase them at a lower price. Then they pocket the difference as profit and return the shares to their owner.
President Bush has identifed Osama bin Laden as prime suspect in the attacks. Experts on Mideast extremists said bin Laden and associates may be capable of sophisticated securities market manoeuvres.