A global manhunt resulted in the arrests of about two dozen people suspected in plotting two other terrorist attacks after Sept. 11, officials said Thursday.
An attack on the U.S. Embassy in Paris and an attack with explosives on a military site in Brussels, Belgium were thwarted by the probe, the officials said.
Evidence seized in raids suggests collaborators were in various stages of planning several other plots to harm U.S. interests in America and abroad, officials said.
The officials, who work in law enforcement and intelligence, spoke only on condition of anonymity.
The FBI, aided by CIA intelligence, have made dozens of raids and searches in the last two weeks.
Information about the planned overseas attacks first emerged this summer, well before the Sept. 11 hijackings, when authorities captured an alleged associate of Usama bin Laden and he began cooperating, officials said.
The alleged bin Laden associate, Djamel Begal, provided overseas authorities with information about possible targets and the names of others who might be involved, officials said.
A senior government official, speaking on condition of anonymity, said U.S. intelligence urged its European counterparts to begin rounding up suspects before Sept. 11.
Spanish Interior Minister Mariano Rajoy said authorities had been watching several of the collaborators for several months but lacked sufficient evidence to move in on them.
After the attacks on the World Trade Center and Pentagon, European police moved rapidly to capture as many of the alleged planners as possible.
They include about two dozen people arrested or detained in Spain, France, Britain, the Netherlands and Belgium. Several other suspected collaborators remain at large, the officials said.
Since the first wave of arrests, authorities have begun to make links between collaborators in different countries.
"We contacted Belgium and said, 'Hey, the guy you arrested was in Spain on such and such a date and met with these guys," Madrid Police Chief Juan Cotino explained. "That's how the link emerged."
Court records suggest three men arrested during a raid at a Detroit apartment earlier this month also had some information about the attack on Belgium.
Authorities have evidence the attack targeted a NATO installation and an American diplomat in Brussels, the officials said.
The thwarted attacks on Paris and Belgium illustrate the current reality facing U.S. authorities — that bin Laden-backed terrorist intended to inflict far more damage on U.S. interests in the coming months.
FBI agents have gathered evidence and testimony — some sketchy, some detailed — suggesting several Middle Eastern men were in various stages of planning, training or exploring targets for additional attacks on U.S. soil.
Law enforcement and government officials caution that some evidence about the future plots is circumstantial at best, but it nonetheless compelled authorities to issue a series of warning in recent days, including that:
— Crop-dusters might be used to spray chemical or biological agents.
— Tanker trucks loaded with lethal poisons could be commandeered.
— U.S. power plants and water supplies could be in danger.
One official said writings and literature seized in some raids suggested the terrorists appeared to have a motive beyond mass casualties — paralyzing the U.S economy.
The Sept. 11 attacks are now seen as part of rapidly expanding terrorist activity among anti-American groups that have been working together loosely in the Middle East and Europe -- with key operatives in the United States. The U.S. government has repeatedly identified bin Laden as the mastermind.
In secret testimony before the Senate Intelligence Committee back in February, CIA Director George Tenet detailed numerous recent attacks that had been thwarted, officials said.
Before he testified in closed session, Tenet hinted at the scope of the attack plans. "We and our allies have scored some important successes against terrorist groups and their plans," he said.
A senior intelligence official said scores of terrorist activities — from meetings to bombings — were thwarted or disrupted over the last few years without public notice. One that has been public was a foiled plan to bomb the Los Angeles Airport during the millennium celebrations.
In most cases, terrorists were simply intercepted at locations in the United States and abroad and stopped on immigration technicalities before they could carry out plans that had been intercepted by U.S. or allied intelligence, the official said.
"There have been in recent years scores of terrorist attacks thwarted or pre-empted, some long before someone get close to lighting the fuse," the official said.
The official declined to give specifics, citing a desire to keep terrorists from knowing how the United States learned of the activities.
The Associated Press contributed to this report.