Theirs was a soft, subtle presence in this country for many months, in some cases years. They didn't blend in, exactly, but they stayed out of trouble.
One suspected ringleader, Mohamed Atta, got a ticket in April for driving without a license and failed to show up for his court hearing, but police never followed up on a bench warrant for his arrest. Other than that his only slip-up came a couple of years ago in Germany, when he failed to return three rented videos ("Ace Ventura," "Vampire" and "Storm of the Century Part II") and the movie-rental company turned to a collection agency.
These conspirators didn't slip into the country in the dark of night. They were welcomed in daylight. At least some, if not all, arrived legally, with visas.
They tended to pay for things with wads of cash. At times they could be standoffish, and make people nervous. Neighbors sometimes had an uncomfortable feeling — why didn't they talk more? Why did this man claim to have no home phone number? Why were these people holding meetings so late?
Even so, as the names of the suspected hijackers have been made public, you don't hear people saying they saw it coming. No one says, "We always thought that guy was a terrorist."
As details of the conspiracy emerge, it's clear the terrorists took great pains to conceal their plans and their identities. They listed nearby Mail Boxes Etc. outlets as home addresses. They moved every two or three months. They may have assumed the identities of others. They skittered across the surface of a large and diverse nation with hardly a snag.
Loners and married men
They were young men, mostly, the youngest just 20. Some were loners. One told his landlord he hoped to find a Mexican bride; they made good wives, he thought. A few were married, with small children who played with American kids in their neighborhoods.
They favored motels and apartments and rented homes in beachfront communities near the flight schools in Florida. Their locations — Delray Beach, Vero Beach, Daytona Beach — have been famous for retirees and race cars, never terrorists.
At times they landed elsewhere: Phoenix, San Diego. One lived briefly just outside the Washington Beltway, taking English classes and computer training.
The businesswoman who runs the classes describes him as "very quiet and gentle."
This is the pattern. "To me, they acted like normal human beings, nothing abnormal," said Henry George, who taught two hijacker suspects, Atta and Marwan Al-Shehhi, in Dade County, Fla., last year. "They were polite, maybe even shy."
Charles Lisa, who rented apartments to two of the men in South Florida, said they were the kind of young men you'd want to take to a baseball game. When they moved out, the landlord asked for a forwarding address. One of the young men smiled and said, "I'll send you a postcard."
One was no apparent zealot
Osama bin Laden, the alleged mastermind of Tuesday's attacks, once said this about Western governments: "They violate our land and occupy it and steal the Muslims' possessions, and when faced by resistance, they call it terrorism."
That's the context in which the suspects in the attacks lived as they made their lethal plans. The Americans around them didn't think they were terrorists. The hijackers didn't think so, either.
At least one didn't fit the image of a Muslim zealot at all. The Lebanese family of Ziad Jarrah, a man named the pilot of the plane that crashed in Pennsylvania, described him as a secular-minded student who had a girlfriend and drank alcohol — not someone who would be inspired to terrorism by Islamic fundamentalism.
"He didn't pray or fast. He never cared about politics or organization. All he cared about is having fun and drinking beer," said an uncle, Nazem. "If he was actually on the plane, then he is one of the passengers, not a hijacker."
The pair from Hamburg
Among the 19 hijackers identified by authorities was Mohamed Atta, a globetrotter, someone who was born in Egypt, received a degree at the Technical University in Hamburg, Germany, and most recently lived in the suburban Fort Lauderdale, Fla., community of Coral Springs. Atta is thought to have piloted American Airlines Flight 11, the first to slam into the World Trade Center.
A letter written by Atta, left in his luggage at Boston's Logan Airport, said he planned to kill himself so he could go to heaven as a martyr. Some reports have said the letter was dated 1996, adding to the evidence that the operation was years in the planning.
In Hamburg he lived with Marwan Al-Shehhi, 11 years his junior. Atta and Al-Shehhi would be largely inseparable for years to come — until the day they boarded separate planes in Boston and hijacked them to New York City.
The chief federal prosecutor in Hamburg, Kay Nehm, said that Atta and Al-Shehhi had organized a terrorist cell in the city "with the aim of launching spectacular attacks on the institutions of the United States."
The family next door
There was one pronounced link between the conspirators and the rest of American society: their children. The kids didn't have to participate in the jihad.
In Vero Beach, Lisa Dubose's 6-year-old son was buddies with the son of a man named Abdul Rahman Alomari — who later boarded American Airlines Flight 11 with Atta.
The adult Alomaris didn't socialize much. A wave now and again. They spent time with another Muslim family — clannish behavior that the American neighbors assumed was normal. Theirs was a nice home, rented for $1,400 a month.
The only problem with the Alomaris were the late-night meetings. Next-door neighbor Betty Egger said that as many as a dozen cars would be parked outside, some on her own lawn. It rattled her to see car headlights flashing through her windows at 2 a.m.
Alomari told his landlord in August that the family would soon be moving back home, to Saudi Arabia. Then, just before Labor Day, something unusual happened: The Alomaris threw a party for all the neighborhood kids.
"They invited all the kids, even ones they'd never seen before," said neighbor Andrew Krease.
Where they come from, Alomari's wife told the neighbors, it's customary to throw a party before moving — to leave nice memories.
The difficult student pilot
Another suspected hijacker, Hani Hanjour, believed to have been the pilot on the plane that hit the Pentagon, appears to have lived in Arizona for the past five years and received pilot training at CRM Airline Training Center in Scottsdale, Ariz., according to company official Gerald Chilton. For three months in 1996 and in December 1997, Hanjour received private pilot training instruction to become a pilot of a single-engine aircraft.
But Hanjour, Chilton said, "never completed the course. He was not believed proficient enough to obtain a license." Then, he said, Hanjour called last year to get more training, this time on multiengine planes. He was rejected.
"He just wasn't a good student with the dedication we see in U.S. Air Force pilots that train there or European airline pilots," Chilton said. "Not that he was rude or impolite. He was just described as a difficult student."
Many details of the plan remain unknown, but its execution began no later than Aug. 25, when the hijackers began buying plane tickets. In many cases they used Internet travel agencies, such as Travelocity.com. Money was apparently no object. Two of the men paid $4,500 each for one-way first-class tickets on United Airlines Flight 175 — putting them close to the cockpit.
As the day of terror neared, the conspirators seemed in high spirits. The previous Friday night Atta, Al-Shehhi and a third man spent hours drinking and playing video games at Shuckums, a Hollywood, Fla., sports bar. Atta played video Trivial Pursuit with great determination.
When a dispute over paying the bill erupted, Al-Shehhi pulled out a wad of cash and said: "There is no money issue. I am an airline pilot."
The terrorists appear to have put greatest emphasis on Flight 11. Multiple hijackers on that plane had flight training. They also went out of their way to bypass security at Logan Airport.
Officials believe Atta and Alomari rented a car in Boston, drove to Portland, Maine, and took a room Monday night at the Comfort Inn. They then flew on a short flight Tuesday morning from Portland to Boston, changing to Flight 11. By going through security at the small airport in Portland — at the groggy hour of 5:44 a.m. — they avoided the tougher security checkpoint in Boston.
Roger Quirion and Vincent Meisner, making business trips to the West Coast, flew with Atta and Alomari on that first flight Tuesday.
"They were joined at the hip," Quirion recalled. The two men struck him as clean-cut, wearing slacks, dress shoes, long sleeve shirts, and carrying dark shoulder bags. Their hair was closely cropped. They had no facial hair. In short, they looked like typical businessmen. Unmenacing.
One of the hijackers took a seat in the fourth row. As Meisner passed to take the seat behind him, his luggage accidentally bumped the hijacker suspect's shoulder.
"Excuse me," Meisner said.
The man merely hunkered lower, putting his head down.
Meisner thought, "Well, he hasn't had his coffee yet, so I'll leave him alone."
It's still unknown precisely how the hijackers took control of the planes. Cell-phone accounts from passengers indicate that in some cases they may have stabbed crew members. Cockpit doors are supposed to be locked, but they are too flimsy to be an obstacle.
In the cockpit they would have faced a number of challenges. Most had nothing like the level of training an airline pilot has. They had to navigate to their targets and override autopilots.
But they had one advantage: They didn't have to know how to land.
Information from The Associated Press is included in this report.