On the Thursday after the terrorist attack, Chicago Board Options Exchange market maker Jon Najarian noticed that trading in options contracts on American and United Airlines stock had been remarkably active just before terrorists hit the World Trade Center.

"It seemed extraordinary," Najarian said later. "Normally, there's next to no volume in those contracts."

The next day Najarian learned there had also been unusual activity in Europe involving sales of shares in Germany's Munich Re insurance company--a company likely to pay out big claims on the disaster.

If you knew in advance about the attack and wanted to make a fortune, Najarian asked one of his partners, "What else would you sell?" Boeing, the colleague replied, and maybe the stocks of Wall Street securities firms. Najarian checked and found the same pattern: Options trading on jetmaker Boeing Co. and the Bear Stearns and Merrill Lynch brokerage firms had risen sharply the week before the attack.

Najarian phoned an exchange official. Breathlessly, he began to describe what he had learned.

"I know," the official said. "The FBI's already been here."

The suspicious trades were part of what traders, securities lawyers and investigators are describing as an extraordinary, unexplained surge in bets against the companies most vulnerable to the attack in the days before it happened. Authorities in at least seven countries are investigating whether people affiliated with the terrorists mounted a sophisticated effort to enrich themselves with more than $100 million in blood money.

Some securities experts caution that it is possible investors had legitimate reasons unrelated to a coming attack for executing the many transactions.

But seasoned investigators such as William McLucas, former enforcement director of the Securities and Exchange Commission, say the trading is suspicious enough to warrant the massive investigation now under way.

"This would get a lot of attention" from law enforcement agencies, McLucas said. It's entirely possible, he adds, that "somebody knew something."

The trades, other experts note, appear to have been executed with a great deal of precision. While options trading rose on United parent UAL Corp. and American parent AMR Corp.-- owners of the planes used in the attack--options on other airlines, such as Delta, Continental and Northwest, traded at normal volumes.

The trades involved "put" options, which entitle holders to sell a certain number of shares of a stock for a specified price during a specified time. Such put options soar in value if a stock's price declines sharply, as occurred on the first trading day after the attack, when United's and American's stock prices plummeted nearly 40 percent.

The Tribune examined trading patterns in American, United, Boeing, Bear Stearns, Merrill Lynch and three other companies whose trading surged during the week before the attack, insurer Marsh & McLennan Cos., oil company Exxon Mobil Corp. and financial-services firm Citigroup Inc., which also has large insurance operations.

All eight companies suffered big stock price drops after the attack. And according to Tribune calculations, options traded the week before the attack in those eight companies could have yielded profits of $163 million or more.

Still, if terrorists tried to use financial markets to profit from the destruction of the World Trade Center, they may find it difficult or impossible to cash in.

The Chicago Board Options Exchange and other financial exchanges around the country reacted almost instantly after the attack, searching for suspicious trades that might indicate someone knew in advance a disaster was imminent and wanted to profit from it.

The Securities and Exchange Commission very likely froze some suspect option trades at the CBOE and other exchanges shortly after the attack, securities lawyers say, although it remains far from certain that those trades were made by terrorists. The FBI is also examining the trades.

Options worth millions of dollars in hard-hit companies like Boeing and United Airlines remain outstanding, with no one having stepped forward to claim the money, traders say. That is highly unusual, as the owners of the options would ordinarily take profits as soon as possible.

The CBOE and SEC declined to confirm or deny that trades were frozen after regulators closed financial markets on the morning of Sept. 11. But in a statement on Wednesday, Stephen Cutler, the SEC's acting enforcement director, took the unusual step of confirming that the SEC is among the agencies investigating the trades.