The search for a connection to Osama bin Laden

But military action against bin Laden seems much more likely than the prospect that he would be seized and returned to the US to stand trial, says Richard Bulliet, a history professor and director of the Middle East Institute at Columbia University in New York. "I think there is a distinction between terrorist acts that fall in the category of criminal activity and those that fall in the category of warlike activity," he says.

From this perspective, the current investigation is much more important to the US for the intelligence it could provide US forces.

Besides the 19 hijackers identified by the FBI, US immigration agents are detaining at least 25 individuals believed to have information about the hijackers. Government officials say some of the detainees are cooperating with the FBI, but the officials decline to quantify the extent of that cooperation.

In addition, agents are working to trace all transactions associated with the hijackers. The basic investigation began with passenger lists obtained from the airlines. "They started with aircraft manifests. That led quite rapidly to the identification of the members of the group that actually went on board the aircraft," says Peter Crooks, a former FBI counterterrorism specialist who is now a professor at Southern Connecticut State University in New Haven. "That gives you a focal point to start moving out."

From there, investigators identified cars rented by the group, and computers and cellphones they used. Documents related to the equipment indicated home addresses and businesses, Mr. Crooks says. From there, agents were able to question neighbors and colleagues.

One particularly critical question is how the hijackers were financed. Credit cards were used for some purchases, and others were paid in cash. But experts say that given the degree of organization, the hijackers' finances were almost certainly laundered to prevent any tracing.

"My guess is that the money trail will lead to the [Arabian] peninsula and disappear in the sands," says Mr. Bulliet.

Yet the investigation is painting a surprisingly detailed portrait of the group of men who allegedly conspired to hijack four commercial jets. At least half of the 19 men identified by the FBI as hijackers are believed to have been natives of Saudi Arabia. The others appear to have come from neighboring countries.

They came to the US apparently as committed members of a terror organization and undertook additional flight training needed to carry out their attacks, terrorism experts say. Several of the men registered at flight schools to learn how to pilot various types of aircraft.

They were well supplied with money, but did not flaunt it in a way that might attract attention. And if they communicated with group leaders outside the US, they apparently did so without being detected by US and other allied intelligence services. Discovering how they may have avoided such detection will be an important part of the investigation, experts say.

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