The FBI has meticulously pieced together a broad terrorist plot, securing evidence the hijackers trained for months or years without raising suspicions in the US, 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 last 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," said Bob Graham, chairman of the Senate Intelligence Committee. "Those acts were going to occur in the United States and elsewhere in the world."
The FBI said it has issued an advisory to fire departments across the country to increase security and guard against the theft of any ambulances or fire trucks, which could be used in bombing attacks. The bureau said the warning was precautionary.
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, the CIA, National Security Agency and other spy agencies.
Evidence sealed
Officials from several of those agencies described developments in the investigation on condition of anonymity. Most of the evidence remains sealed by court orders. A federal grand jury in White Plains, New York, was convened last week to weigh evidence and issue subpoenas.
US 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 US President George W. 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.
Investigators have explored whether the hijackers may have had help from people with access to airlines. On Tuesday, authorities arrested and charged three men in the Detroit area with possessing false documents after a raid on a home where agents found airport-related diagrams and documents about a military base and a US foreign minister.
Authorities said they believe some of the men may have worked at one time for a company that provides food service to airlines at the Detroit airport.
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, New Jersey, 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 toward Texas, where they were taken into custody. They had US$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's attacks, agents had already gathered evidence tracing Moussaoui to an effort to get flight training as early as fall of last year in Norman, Oklahoma, officials said.
Similarly, the FBI has traced the steps of the 19 known hijackers to flight schools across the country, from Maryland to Florida.
Terrorist profile
The FBI is seeking as many as a dozen others who fit this profile: Middle Eastern men who came to the US, 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 US 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 US 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 US -- 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.
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