They traveled the world, often in pairs, studying and working in Europe and the United States. Mostly in their 20s, they came from secular, middle-class Arab families and blended well into Western society - hardly the profile of Islamic zealots plotting the worst terrorist attack in history.
While many details surrounding their daily lives have been discovered, these 19 Arab men remain an enigma. While they are assumed to have been driven by hatred for America, there is scant concrete, publicly known evidence of their mindset, the means by which they were recruited and the point at which they were told about the mission.
Although 15 of the hijackers were from Saudi Arabia, federal prosecutors have alleged in court documents that it was Mohamed Atta of Egypt, Ziad Jarrah of Lebanon and Marwan Al-Shehhi of the United Arab Emirates who "formed and maintained an al Qaeda terrorist cell in Germany" in the late 1990s.
The three Sept. 11 pilots lived in Hamburg where they studied at universities and worked at a computer-packing company. They socialized with the local Muslim community, attending mosques and community celebrations. Jarrah even had a girlfriend. A day before he hijacked United Airlines Flight 93, he wrote her a farewell letter, telling her he wouldn't be back.
In the days after Sept. 11, Jarrah and Atta's families refused to believe what they were hearing. But as their sons have failed to materialize, they have had little choice but to accept the truth.
Mohamed Alshehri's two sons, Waleed and Wail, were with Atta aboard American Airlines Flight 11, which crashed into the north tower of the World Trade Center. "If that turns out to be the truth, then I'll never, never accept it from them. I'll never forgive them for that," the Saudi has said of his sons.
U.S. officials believe the brothers trained in Afghan camps before heading for Florida. A drug store owner in Delray Beach remembered them buying soda and candy bars days before the attacks.
Sticking close together, the Alshehri brothers stayed with each other in Florida; Atta and Al-Shehhi, who were cousins, traveled together across the United States; Khalid Almidhar and Nawaf Alhazmi held a meeting in Malaysia with a suspect in the USS Cole bombing. When the meeting was over, the two flew together to San Diego.
According to federal court papers, 13 of the hijackers entered the United States between April 23 and June 29, 2001. Once in America, they crisscrossed the country living on-and-off in suburban towns with friendly names - Lemon Grove, Calif., Laurel, Md., Deerfield Beach, Fla.
Over the next 18 months, they spent time in a dozen states including Nevada, New York, New Jersey, Oklahoma, Arizona, Virginia, Maine and Massachusetts. Seven were pilots and several others studied or visited flight schools around the country.
Most entered the United States legally but aroused suspicions wherever they went.
Atta abandoned a plane on a Miami tarmac instead of parking it properly; Hani Hanjour said he had 600 hours of flight experience and a valid pilot's license, but flew so poorly that instructors at a Maryland flight school wouldn't let him go solo. One month later, Hanjour was with four other hijackers aboard American Airlines Flight 77, when it crashed into the Pentagon.
The hijackers bribed motor vehicle employees to obtain state driver's licenses. They rented apartments, opened bank accounts, took out gym memberships, bought airline tickets online and walked around with wads of cash. They took flight training by day, went to bars at night, even got speeding tickets.
Overseas, several of them spent time in Pakistan, Afghanistan, Germany, Spain, Malaysia and the Czech Republic where authorities maintain that Atta met with an Iraqi spy.
Nothing has publicly tied the hijackers to the anthrax attacks although they were in close proximity to the American Media Inc. office, site of the nation's first fatal anthrax infection in October. A Florida pharmacist also said he treated Atta for a burning sensation on his hands.
What little doubt U.S. investigators had that 33-year-old Atta was the ringleader was all but removed by a videotape that captured Osama bin Laden saying that Mohamed "was in charge of the group."
As for the men who carried out the attacks, bin Laden said, "we did not reveal" the plan until "just before they boarded the planes." The videotape is the strongest publicly known evidence linking the hijackers directly to the al Qaeda chief.
There is no evidence that any of the hijackers knew Zacarias Moussaoui. But Moussaoui, the only man charged in the United States in connection with Sept. 11, seemed to know some of the same people as the hijackers, replicated some of their movements and attended a couple of flight schools.
U.S. officials believe Moussaoui could have been training for Sept. 11 or a similar mission. But the missing 20th hijacker was likely Ramzi Binalshibh, a Yemeni who lived in Hamburg with Atta and who failed four times to get a U.S. entry permit. Binalshibh wired money to Moussaoui, a Florida flight school where Jarrah was training and to at least one hijacker. Authorities believe Binalshibh fled Germany for Pakistan around Sept. 11.
Much is known about the movements of the Hamburg cell. Less is known, even a year later, about some of the younger Saudi hijackers such as 26-year-old Satam Suqami who was on Flight 11 from Boston with Atta and the Alshehri brothers. Mohald al-Shehri, who was on Flight 175 from Boston, was apparently unrelated to the other two Al-Shehris.
The FBI has determined that Nawaf Alhazmi and Salem Alhazmi, both on Flight 77, weren't related, a U.S. law enforcement official said. And three hijackers with the last name Alghamdi were also unrelated. An independent Arabic translator, a native Saudi, has said bin Laden uttered the name Alghamdi several times on the videotape.