WASHINGTON -- The FBI has meticulously pieced together a broad terrorist plot, securing evidence the hijackers trained for months or years without raising suspicions in the United States, received financial and logistical support from others and identified additional targets for destruction.
Law enforcement and other officials familiar with the evidence said the FBI is investigating whether the terrorist network behind Tuesday's attacks targeted more flights for hijacking beyond the four that crashed.
Authorities have grown increasingly certain -- from intelligence intercepts, witness interviews and evidence gathered in hijackers' cars and homes -- that a second wave of violence was planned by collaborators. They said Sept. 22 has emerged as an important date in the evidence, but declined to be more specific.
Tuesday's attacks were ''part of a larger plan with other terrorism acts, not necessarily hijacking of airplanes. Those acts were going to occur in the United States and elsewhere in the world,'' said Sen. Bob Graham, D-Fla., chair of the Senate Intelligence Committee.
The investigation, the largest in American history, has engulfed the full resources of the FBI, Justice Department, Customs Service, Treasury Department agencies that track assets and the Central Intelligence Agency, National Security Agency and other spy agencies.
Officials from several of those agencies described developments in the investigation to The Associated Press on condition of anonymity. Most of the evidence remains sealed by court orders. A federal grand jury in White Plains, N.Y., was convened last week to weigh evidence and issue subpoenas. U.S. officials have made no secret they believe exiled Saudi millionaire Osama bin Laden masterminded the plot from Afghanistan and organized his and other terrorist groups to carry it out. In President Bush's words, bin Laden is wanted ''dead or alive.''
The FBI has hinted at the magnitude of the collaboration, sending airlines, local police and border patrol agencies a list of about 200 people it believes may have information or assisted the attacks. The government has detained 75 people for questioning and on immigration charges, from California to Germany.
At least four people on the list have been arrested as material witnesses, law enforcement officials said Tuesday. That means they are believed to have critical information about the plot and are at risk to flee.
Several detainees have been flown to New York, where the grand jury is working and where prosecutors have significant anti-terrorism experience from earlier cases involving bin Laden.
These detainees include Ayub Ali Khan, 51, and Mohammed Jaweed Azmath, 47, two men who left the Newark, N.J., airport aboard a flight headed for Texas about the same time as the hijackings. The men were grounded in St. Louis and then took a train to Texas, where they were taken into custody. They had $5,000 cash and box cutters like those used by the hijackers, immediately drawing the attention of law enforcement.
Authorities also have flown to New York a French-Algerian man who was detained last month after he sought flight training in Minnesota. The school where he offered to pay for the training was suspicious, and called authorities. The government has held Zacarias Moussaoui on immigration charges since Aug. 17. Two weeks before Tuesday attacks, agents had already gathered evidence tracing Moussaoui to an effort to get flight training as early as fall 2000 in Norman, Okla., officials said.
Similarly, the FBI has traced the steps of the 19 known hijackers to flight schools across the country, from Maryland to Florida. The FBI is seeking as many as a dozen others who fit this profile: Middle Eastern men who came to the United States, got pilot licenses or sought flight training, like the men who flew jetliners into the World Trade Center and Pentagon.
''We want to know whether there were other pilots, other teams who were supposed to take down airliners or strike Americans in other ways,'' one law enforcement official said.
Agents are investigating whether some associates of the 19 hijackers planned or did board other planes, possibly with similar plans for suicide hijackings that weren't carried out.
Vice President Dick Cheney hinted at such additional hijackings during a TV appearance Sunday when he said U.S. authorities believed six planes were targeted by the hijackers last week.
Law enforcement has gathered evidence suggesting the plot was patiently hatched over many months and years, and that the terrorists spent significant time training for it and grooming supporters.
Many of the hijackers trained or sought training in flight schools as early as 1999, and most entered the United States with legal visas. Some of the hijackers met with supporters overseas, in places like Germany and Malaysia, before returning to carry out their plan, officials said.
''One of the keys to understanding this is the length of time these hijackers spent here. These weren't people coming over the border just to attack quickly. ... They cultivated friends, and blended into American society to further their ability to strike,'' one investigator said.
Authorities said the fact that some of the men claimed to have connections to Middle Eastern countries friendly to the United States -- Saudi Arabia, Egypt and United Arab Emirates -- may have lessened suspicion. Some of the pilots carried identification suggesting they were connected with Saudi Arabia's national airline.
The FBI has pressed for evidence across the globe as to who may have assisted the hijackers, seizing bank and computer records and studying credit cards used to pay for plane tickets, rental cars and the like.
A doctor in San Antonio, where the two Newark, N.J., passengers were heading, has been detained, as has a man in California who has been linked through financial transactions to hijackers Khalid al-Midhar and Nawaf Alhamzi, authorities said.
Al-Midhar and Alhamzi were placed on a watch list this summer after U.S. intelligence received information they might have been meeting with suspected terrorists. By the time they were added to the watch list, they'd already entered the United States, officials said.
On the financial trail, the Securities and Exchange Commission has received information from other U.S. regulators about possibly suspicious trading ahead of the attacks. European regulators are looking to see if bin Laden's network sought to profit off investments related to the attacks.
The potential collaborators are also being linked by communication intercepts -- some of which have occurred since the attacks, authorities told AP.
Those familiar with the investigation say the collaborators have communicated by cell phones that were frequently rotated and by e-mail. They also made calls on traditional phones but may have used a special telephone company code involving the pound sign to make it harder to follow their tracks, officials said.
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