Before Sept. 11, much was suspected, but little known about Al Qaeda. Investigations and recent arrests in Europe and the US, however, are now revealing a clearer portrait of Osama bin Laden's hydra-headed network of operatives and sympathizers.
"We are learning that many assumptions that we operated with are out of the window," says Richard Shultz, a terrorism expert at Tufts University in Boston.
Al Qaeda's level of organization is one of the biggest surprises for those tracking terrorist groups, who now recognize it as potentially a sophisticated, disciplined, and thorough outfit.
Thousands of men who passed through Al Qaeda training camps in Afghanistan have returned to their homes around the world. Many are believed to stand ready for action if called upon. Most of them are not full-fledged members of Al Qaeda, but belong instead to local groups with ties to Mr. bin Laden's organization.
"They are not micromanaged, but that does not mean they are not connected," says Prof. Shultz, whose work with US government agencies has given him access to classified material.
As more is learned about the international terrorist underground, he says, "we will see a much greater set of connections" between different groups, up-ending conventional wisdom of recent years in counter-terrorism circles that terrorist organizations were operating in less and less coordinated fashion.
Already evidence has emerged of Al Qaeda ties - financial and personal - to dozens of organizations in dozens of countries, from Malaysia, Indonesia, and the Philippines, through Kashmir, Chechnya, and Bosnia to Egypt, Algeria, and Somalia in Africa and to France, Spain, and Italy in Europe.
Sometimes such groups have made their alliance with Al Qaeda public, as when a number of them co-signed bin Laden's 1999 fatwa declaring war on "Crusaders and Jews." Sometimes they have tried to keep their relationship secret.
"They have not been subsumed into Al Qaeda" says Neil Partrick, a Middle East specialist with the London-based Royal United Services Institute. "They work with Al Qaeda," giving it "a degree of rootedness in their countries," which became clear in last year's attack on the USS Cole in Aden, and the 1998 bombings of US embassies in Kenya and Tanzania. "We are looking at a loose confederation with various assignments given to various groups," he says.
The failed 1993 bomb attack on the World Trade Center had long been seen as evidence that terrorist groups were increasingly amateur, says Prof. Shultz. Sept. 11 destroyed that perception.
It is now clear that the Sept. 11 attack was planned over as many as two years, coordinated, financed and put into motion from Afghanistan, the United Arab Emirates, Saudi Arabia, Germany and finally the United States.
"Al Qaeda engages in detailed planning and reconnaissance to the point of meticulousness" says Bruce Hoffman, a Washington-based terrorism expert with the Rand Corporation. One telling detail: In the final days of their lives, just before they flew airliners into targets in Washington and New York, the hijackers wired unused funds back to the man in the United Arab Emirates who had financed them, according to the indictment issued last week against Zacarias Moussaoui, an alleged plotter.
"The Al Qaeda training manuals are not rocket science, but they show good, solid tradecraft," says Shultz.
Not all operations are so well funded. Ahmed Ressam, the Algerian planning to set a bomb at Los Angeles airport around the turn of the millennium, told interrogators he had been told to fund himself by petty thievery. A Spanish judge who has indicted eight suspected Al Qaeda members for involvement in Sept. 11 says they financed themselves largely through credit card fraud.
Police in Italy are looking into suspicions that Al Qaeda cells there made money from commissions on international money transfers by immigrant workers. Most of those arrested in three sweeps since last April had menial jobs as cleaners or construction workers.
Some of those detained, say Italian police, had planned to bomb Strasbourg cathedral during millennium celebrations, in an operation that appears to reveal another of Al Qaeda's methods.
While the World Trade Center attack seems to have been coordinated - at least initially - from Al Qaeda headquarters, in a hierarchical manner, the aborted Strasbourg attack was a locally inspired idea that later enjoyed Al Qaeda support and money, according to Italian police.
"Al Qaeda works on multiple levels, which is what makes it such a formidable opponent," says Rand's Mr. Hoffman. "Sometimes it operates top-down, with orders coming from the CEO, and sometimes it is a venture-capitalist operation, from the bottom up, when terrorists come to ask for finance from bin Laden."
Sometimes the operatives are able - like Mohammed Atta, who is seen as the ringleader of the Sept. 11 plot - and sometimes they are less skilled. Mr. Ressam, for example, panicked when he was stopped at the US-Canadian border and tried to run away. Later he cooperated fully with FBI investigators.
How Al Qaeda operatives are recruited is still not entirely clear. In some countries, such as Spain, judging from the indictment, mosques are natural recruiting grounds, but this is not always the case.
A lot of the spotting was done in Europe, where a number of countries have been revealed as convenient and anonymous places for terrorist cells to organize. Police have made arrests in connection with Al Qaeda in France, Britain, Spain, Italy, Holland, Belgium and Germany over the past three months, though only a handful of the suspects have been charged.
Nobody believes that authorities have rolled up all the groups linked to Al Qaeda, and US Attorney General John Ashcroft has warned that 'sleeper' cells may still be intact in the United States.
The most critical piece of missing intelligence on Al Qaeda, says Hoffman, is "where they are going to strike next. We just don't know what's up their sleeve."
Sara B. Miller in Madrid and Courtney Walsh in Rome contributed to this report.