So far, the story of the suspects is a maddening maze of sketchy records, aliases, and apparently phony documents. Even now, the FBI is uncertain about the true identities of a few of the purported hijackers, leaving agents to work from the list they assembled from flight records.
There were Ahmed Alnami, 23, and Saeed Alghamdi, 21, who stopped at the Mile High Travel agency in Lauderdale-by-the-Sea, Fla., on Sept. 5 to book a flight to Newark, N.J., four days later. An employee at the agency remembers them as bargain-hunters. They found a terrific buy: one-way fares of $139.75 each with just two days' advance purchase.
There were the four men in their 20s with foreign accents and an aloof manner who shared an apartment this summer on Dorttel Road in Delray Beach, Fla. A neighbor, Stacy Warm, thought they were drug dealers because they came and went at odd hours, carrying dark bags.
"They were extremely unfriendly," she said.
Then there was Mohamed Atta.
The son of an upper-middle-class Egyptian family, he was described as intellectual and clear-thinking by a professor at the German university where he wrote an A-plus thesis. Yet he did something stupid one day while learning to fly in Venice, Fla. After stalling a Piper aircraft on a runway, he should have radioed the tower. Instead, he jumped out and strolled toward the terminal.
"You don't do that; you don't walk across an active taxiway," said Dale M. Kraus of Huffman Aviation, where Atta was taking his lessons.
There is one snapshot: a color photograph taken by a security camera at the Portland, Maine, airport at 5:45 a.m. on Sept. 11.
It shows Atta and a man believed to be Abdulaziz Alomari just after they passed through metal detectors. They were bound from Portland to Boston, where at 7:45 a.m. they would board American Airlines Flight 11 - the first plane to crash into New York's World Trade Center towers. Investigators speculate that they went to Portland first because they believed it would be easier to get through security there carrying box cutters.
Atta is in the lead, carrying a dark bag over his left shoulder and wearing a blue button-down shirt. His expression is alert, focused. Behind him comes Alomari - younger, slimmer, in short sleeves. His head down, he, too, carries a bag.
Investigators say they believe some of the hijackers stole identities from people, living or dead, in Saudi Arabia. In that country, the surname of Alghamdi, which was used by three of the men, is "as common as Smith," a Saudi Embassy spokesman in Washington said.
What has become clear is that the terrorists lay patiently in wait for months, even years. Their movements were like flashes in darkness.
In months and years leading up to the attacks, the hijackers moved around like the transient students or low-wage workers that they often seemed to be. Investigators have picked up their footprints and those of apparent accomplices in Florida, California, New Jersey, Maryland, Virginia, Texas, Minnesota and Michigan.
The trail also has led to Hamburg, Germany, where three of the purported terrorists attended universities in the late 1990s. Among them was Atta.
Older and better educated than others accused in the hijacking, Atta, 33, appears to have carried himself as something of a leader within the group.
Described as an extremely serious student at the Technical University of Hamburg-Harburg, he produced a 152-page thesis on the renewal of the old quarter of Aleppo, Syria, that showed him to be a "well-reasoning person," said his academic adviser, Dittmar Machule.
The scholarship betrayed no biases, Machule said. It called for Christians, Jews and Muslims to live together. But restlessness trembled beneath Atta's reserve.
"He was a searcher in his religiosity - like somebody who despaired at the world," Machule said, "somebody who combined high intellect with deep faith."
On the second page of the thesis, Atta quoted a passage from the Koran: "Speak: my prayer and my sacrifice and my life and my death belong to Allah. The lord of the worlds."
Unknown to his German hosts, Atta may have taken part in a bus bombing in 1996 in Saudi Arabia, investigators say. Militants there were angry at the U.S. military presence on what they saw as holy Islamic soil.
Hamburg, a wealthy city on the north German plain that was virtually destroyed by a British firebomb attack in World War II, also attracted Marwan al-Shehhi and Ziad Jarrahi.
Jarrahi attended Hamburg College of Applied Science, studying aircraft construction. Shehhi joined Atta at Technical University, which since Sept. 11 has been referred to by some in Germany as "Terrorist University."
Hundreds of investigators have descended on Hamburg since learning that the three suspected hijackers had lived there between December 1998 and February 2000 in a furnished three-room apartment at Marienstrasse 54 on the city's southern edge.
A key piece of the puzzle is the man who signed the apartment lease: 26-year-old Said Bahaji, a German citizen of Moroccan ancestry and an electrical engineering student at the Technical University. Bahaji may have provided visas, money and apartments, a Hamburg police official said.
On Friday, international warrants were issued for Bahaji and Ramzi Binalshibh, a 29-year-old Yemeni national. Kay Nehm, Germany's chief prosecutor, said both were wanted for "building a terrorist organization."
Neighbors of the three-room apartment noted how little noise the men made, even when as many as 20 people visited. Men would come and read the Koran, leaving their shoes outside.
One neighbor's curiosity got the better of her. Each night about 10:15, Monika van Minden says now, she stood at her kitchen window, smoking a cigarette as she cooked her husband's nightly egg. The 52-year-old homemaker gazed across the narrow street at the men in their second-floor apartment, furnished with little more than computers, and saw them sitting in a circle, drawing on sheets of paper. Then they tacked the sheets to the walls. When they caught her watching, they hung blinds.
The wife of Said Bahaji, the man who handled the money, has told police he left for an internship in Pakistan before the attacks, and authorities say he flew on Sept. 3 to Istanbul and then on to Pakistan. The German prosecutor, Nehm, said that on Aug. 30, Bahaji gave his wife's stepfather power of attorney - in case something happened to him.
The captors of American Airlines Flight 11 appeared to have gone to the greatest lengths to guarantee a smooth operation.
Four of the five had undergone pilot training in the United States. They also took the precaution of flying into metropolitan Boston from the smaller, less scrutinized Portland airfield.
Before the 7:59 a.m. takeoff of Flight 11, Atta and Alomari were comfortably seated in Row 8, first class, of the Boeing 767, bound for Los Angeles. Across from them sat David Angell, executive producer of the television show Frasier.
The three other men - Satam al-Suqami and a pair of brothers, Waleed and Wail Alshehri - were also on board.
Atta had bought his ticket on the airline's Web site a few days earlier, with a newly opened frequent-flyer account.
While in the United States, the men on the FBI's hijacker list tended to live like Cold War spies, staying in cheap motels, keeping to themselves.
Atta, though, attracted some attention. In April, he was ticketed in Florida for driving without a license. When he did not show up for his court appearance, a bench warrant was issued for his arrest. But it was a routine driving offense, and there is no record of Florida police stopping him again.
The hijackers all came from countries with strong Islamic traditions, but they appeared to take part in Western decadence at times. There were stories of them carousing at Florida bars. One hijacker may have left a Koran at a strip club in Daytona Beach. Some appeared to have gambled in Las Vegas.
Their footprints are widely scattered.
While in Florida in May and June, Atta lived at the Tara Gardens apartment complex in Coral Springs. Diana Padilla, who lived upstairs, recalled: "You would say hello to him, and nothing - no reaction."
Atta hung around mainly with Marwan al-Shehhi, whom authorities now say they believe helped hijack United Airlines Flight 175, the plane out of Boston that hit the south tower of the trade center.
They took pilot training together at Huffman Aviation then enrolled in two three-hour courses at SimCenter Inc. in Opa-Locka, Fla.
At the Opa-Locka school, they trained on a Boeing 727 full-motion flight simulator and concentrated on turns. They did not learn to land.
By last spring, Waleed Alshehri, one of the Flight 11 suspects, was staying at the Bimini Hotel on Hollywood Beach in Florida. He obtained a Florida driver's license last May using the hotel address. His brother Wail also stayed there.
Hotel owner Joanne Solick said they told her they were in town to get pilot's licenses.
At the end of last June, Waleed Alshehri moved to the Homing Inn in Boynton Beach, Fla. There he was joined by Wail Alshehri. Also there was Suqami, about whom little is known. The FBI says he was in the Flight 11 group.
Fifteen minutes after American Flight 11 left Logan, United Airlines Flight 175 lifted off from the same airport. Like the earlier flight, the Boeing 767 was Los Angeles-bound. But it became the second jetliner flown into the trade center's towers.
Again, five hijackers were aboard, the FBI says. The plan appeared to call for five-man teams on all four flights. Only the plane that crashed in Pennsylvania, after taking off from Newark, N.J., had fewer than five. Investigators speculate that a man arrested earlier for an immigration-law violation was supposed to be the fifth on that flight.
Investigators say Marwan al-Shehhi lived at a rooming house in a Virginia suburb of Washington. Justice Department documents list three Virginia addresses that some of the hijackers may have used.
Like nearly all the hijackers, the Flight 175 suspects - identified, besides Shehhi, as Ahmed Alghamdi, Mohald Alshehri, Hamza Alghamdi and Fayez Ahmed - left tracks in Florida.
With a fast-growing and international population, Florida is one of the easiest states in which to acquire a driver's license. It also has many flying schools.
While in Florida, the eventual hijacking suspects did research on public library computers. One library was in Delray Beach, where director John Callahan recalled that they put their names on sign-in sheets to wait for one of 14 public-access terminals.
A research librarian, Kathleen Hensman, remembered one of the names: Mohald Alshehri, from Flight 175.
While Marwan al-Shehhi and Atta were taking lessons from Huffman Aviation beginning in July 2000, they paid $15 per day to stay in one room of the house in Venice owned by Charlie Voss and his wife, Dru.
Federal officials estimated last week that the terrorist operation might have cost as much as $200,000, though some investigators are saying it could have been done far more cheaply. Whatever the case, much of the money went into flight training and travel, investigators say. It surely did not go into living expenses.
From Aug. 26 to Sept. 9, just before the attacks on the World Trade Center and the Pentagon, Shehhi and an unidentified man rented a cheap room at the Panther Motel in Deerfield Beach, Fla.
Owner Richard Surma said they were good tenants. "No noise, no loud music," he said.
They paid $250 a week in cash. After they left, Surma discovered a canvas tote bag in a motel trash bin. Inside the bag he found an instruction manual for flying a Boeing 757, a German-English dictionary, a fuel tester, and maps of the eastern United States.
It seemed odd, but he thought nothing further of it.
In Laurel, Md., tucked away in a downscale motel, the men suspected of commandeering American Airlines Flight 77 and crashing it into the Pentagon lived out their final days.
At least two - and possibly all five - bunked together in Room 343 at the Valencia Motel, an 80-unit complex with an orange-and-brown stone facade.
Rooms are rented by the week or longer. The manager, Rakesh Shah, said he rented the room to them from Aug. 23 to Sept. 11.
That morning, Flight 77 was to leave at 8:10 from Washington's Dulles International Airport, headed to Los Angeles. The men paid their $308 bill with a credit card.
The names of two of the men, Khalid al-Midhar and Nawaq Alhamzi, became known to U.S. authorities long before Sept. 11.
Midhar had reportedly been seen last year in Kuala Lumpur, Malaysia, meeting a suspect in the terrorist bombing of the USS Cole. Seventeen U.S. servicemen were killed in the attack off the coast of Yemen in October 2000.
Alhamzi and Midhar lived for a time last year in San Diego. Midhar reportedly entered the country July 4 on a flight from Saudi Arabia to New York. He had a business visa.
Sometime over the summer, Alhamzi - along with Salem Alhamzi, another of the men aboard Flight 77 - left a trail in Fort Lee, N.J., just outside New York City. For $60, they rented a mailbox at a Mailboxes, Etc. store in a mall, law enforcement officials say.
Midhar and Nawaq Alhamzi were on a U.S. "watch list" of suspected terrorists. The lists are supposed to warn law enforcement. If the men had been found, they might have been placed on 24-hour surveillance.
But like the others, they kept to themselves. Gail North, a housekeeper who used to work at the Valencia, said she was never allowed inside their room to change the towels. They passed the soiled ones to her through the door.
Hani Hanjour, possibly the pilot of Flight 77 after it was hijacked, took lessons last month at the Freeway Airport in Bowie, Md. But after he made some awkward loops over the airport and the Chesapeake Bay, the instructors refused to pass him.
The Pentagon suspects, or at least several of them, were last seen in Laurel on Sept. 10, packing their car about 10 a.m. outside the Valencia Motel. North remembered it, she said, because it was her wedding anniversary.
Twenty-three hours later, at 9:43 a.m., Flight 77 crashed into the Pentagon.
The last of the jetliners to go down was United Flight 93, a Boeing 757 that took off from Newark, N.J., at 8:01 a.m. and was in the air for an hour and nine minutes before crashing into the countryside of rural Pennsylvania.
On board was a member of the Hamburg group: Ziad Jarrahi, 26.
It was on this flight that passengers called loved ones on cellular phones to say they had banded together to try to prevent the hijackers from aiming the craft like a flying bomb in Washington. Jarrahi is thought to have been at the helm.
Jarrahi had grown up in the Bekaa Valley of eastern Lebanon, which in the 1980s was the base of Iranian- and Syrian-backed terrorist groups that attacked American and Israeli interests in the region.
Like his mentor, Atta, Jarrahi was a Sunni Muslim, but he had gone to a Christian school. Unlike the poor and disenfranchised youths who often gravitate to extremist groups, he came from a respectable family. His father was a civil servant; his mother, a teacher.
In 1997, Jarrahi went to study aircraft engineering at Hamburg's University of Applied Sciences. He soon found a girlfriend, Aysle Senguen, 25, a Turkish medical student.
After the hijackings, in an interview with the German newspaper Bild, she recalled that Jarrahi had grown stern with her about her modern ways.
"In the end," she said, "Ziad and I were fighting a lot. He wanted me to wear a head scarf. I wasn't allowed to listen to Western music and shouldn't go to parties anymore. All of a sudden, he wanted me to live strictly according to the Islam."
Last April, Jarrahi showed up in Florida.
He lived in a furnished, one-bedroom apartment on Harding Street in the town of Hollywood until June 23. His roommate was Ahmed Alhaznawi, whom the FBI says was also aboard Flight 93.
Jarrahi also may have had Florida contacts with the others accused of hijacking Flight 93: Saeed Alghamdi, 21, and Ahmed Alnami, 23, who took the $139.75 flight to Newark, on Sept. 9.
Jarrahi obtained a Florida driver's license in May. He bought a red 1990 Mitsubishi and registered it at the Hollywood apartment, which rented for $165 a week.
From Hollywood, Jarrahi and Alhaznawi moved to a house on Bougainvilla Drive in Lauderdale-by-the-Sea. Their landlord said the pair showed him what appeared to be German passports as identification.
"They said they were pilots taking flying lessons at one of the airports around here," landlord Charles Lisa, 64, said. "They had quite a few visitors who used to walk up to the apartment.
"When they left, I asked them for a forwarding address. But Ahmed just smiled at me and said, 'I'll send you a postcard.' "
Tom Infield's e-mail address is firstname.lastname@example.org.
Knight Ridder reporters David Goldstein, Alan Bjerga, Frank Lockwood, Curtis Morgan, David Kidwell and Oscar Corral and Peter Nicholas of the Inquirer Washington Bureau contributed to this article.