At the age of 24, Mohamed Atta, the suspected ringleader of the Sept. 11 plot, travels to Hamburg, Germany, and enrolls at a technical university to do graduate study in urban planning. Shortly after his arrival in Hamburg, Atta, already a qualified architect, took a part time job with a local design firm.

Atta begins attending Al Quds mosque, where young men from many Arab countries congregate. Investigators believe Al Quds mosque is where Atta was radicalized, either through meeting extremists or by direct recruitment by an agent of bin Laden. They also believe Atta met two other young Arabs at Al Quds who would become members of the Hamburg terror cell and, later, the suicide pilots on Sept. 11 -- Ziad Jarrah and Marwan al-Shehhi.

Atta grows a beard during this time, a sign of his growing fundamentalism. He also writes out a will, which is witnessed by some of his friends at the mosque. He urges them not to be sad when he dies.

In April 1996, Ziad Jarrah leaves Lebanon for Greifswald, Germany, where he studies German for two semesters before qualifying to take other university courses. After a year in Griefswald, Jarrah continues his studies at the University of Applied Science in Hamburg, where he stays until 1999. Once in Hamburg, Jarrah rents a room from a German landlady in a genteel suburb. Jarrah begins dating -- and then living with -- a Turkish-German woman, a secular Muslim like himself, who is about to start medical school.

In Hamburg, Jarrah begins a college course in aircraft engineering. After a few months, his landlord, Rosemary Canel, notices small changes that suggest a new commitment to Islam. According to Canel, he grows a beard and keeps a small prayer mat in his room. He also starts spending time at the Al Quds mosque with al-Shehhi and Atta.

Marwan al-Shehhi comes to Germany from the United Arab Emirates (U.A.E.) in 1997. During this time, he studies German at the University of Bonn. According to press reports, al-Shehhi moved to Hamburg sometime in 1999, where he enrolls at the Technical University.

Atta disappears from his university for over a year, giving only a vague explanation for his absence. Western intelligence has established that Atta traveled to Afghanistan, to the camps of Al Qaeda. He is followed by al-Shehhi and Jarrah.

While Atta is in the camps, Osama bin Laden makes his most menacing threat against the United States. He declares all Americans to be legitimate targets of terror. Atta also is in the camps when bin Laden's men bomb two U.S. embassies in East Africa, killing 224 civilians, including 12 Americans.

Atta reappears at his university in Hamburg. Observers say he looks more serious, works hard, and doesn't smile as much as before. By now, he has become an operational commander for bin Laden, and is preparing a big attack. Following the blueprint set out in Al Qaeda's manual, he forms a terror cell. He finds an apartment, No. 54 Marienstrasse, that serves as a base for the cell. The apartment becomes the meeting place for at least six Arab students, including al-Shehhi and Jarrah.

German intelligence misses an opportunity to detect the cell and spot their plot. In the aftermath of the Nairobi bombing, the Germans carry out surveillance on one of Atta's roommates who is suspected of having links to bin Laden. The investigators never focus on Atta or the other future pilots.

Atta, al-Shehhi, and Jarrah all report their passports stolen. The old ones bore the evidence that they had all been to bin Laden's camps. They apply for and receive new passports.

Meanwhile, Atta's life in Germany is drawing to a close. He submits his master's thesis on urban renewal in an ancient Islamic city. It earns the highest possible grade.

Al-Shehhi, Atta, and Jarrah arrive separately in the U.S. on tourist visas from Berlin.

Al-Shehhi flies from Europe to Newark, N.J., on May 29, 2000. According to the indictment against Zacarias Moussaoui, an alleged co-conspirator in the Sept. 11 plot, one month after al-Shehhi enters the U.S., $4,790 is wired to him from the U.A.E.

Jarrah arrives in Atlanta, Ga., on a June 27 flight from Munich, Germany.

Atta arrives in Newark on or about June 3, on a tourist visa he had received in Berlin on May 18. Investigators say that Atta, instead of traveling directly to the U.S., traveled to Prague in the Czech Republic, where he stayed for a day before traveling to Newark.

The trio quickly make their way to Florida to learn how to fly. Atta and al-Shehhi enroll at Huffman Aviation. As foreigners, they need student visas to take lessons, but they are accepted even though they both have only tourist visas.

Atta and al-Shehhi receive flight training at Huffman Aviation in Venice, Fla., where they pay $38,000 for lessons. According to the indictment against Moussaoui, from July 19 to September 18, $109,440 is transferred from the U.A.E. to a Florida SunTrust bank account under the names of Atta and al-Shehhi. (The FBI says Atta and al-Shehhi eventually were sent more than $100,000 each -- most of the money coming from Dubai, in the U.A.E., one of the world's busiest and most anonymous banking centers.)

Jarrah begins attending the Florida Flight Training Center in Venice, Fla.

He also tries to enroll another member of the Hamburg cell, Ramzi bin al-Shibh, in flight school. At the time, bin al-Shibh is still living in the Marienstrasse apartment. Bin al-Shibh is from Yemen, however, which is considered a hotbed of terrorism, so the U.S. denies his visa four times. The FBI now believes that bin al-Shibh was meant to be the fourth hijacker pilot.

According to the indictment against Moussaoui, Atta purchases flight-deck videos for Boeing 747 and 757 aircraft in a pilot store in Ohio on or about Nov. 5, 2000. He purchases more videos a little over a month later, this time for Boeing 767 and Airbus A320 aircraft.

Atta and al-Shehhi visit a jet aircraft simulator in Opa-Locka, Fla. For $1,500, they buy six hours of simulator time. They say they want to join an airline and need to familiarize themselves with an airliner cockpit.

On Dec. 26, 2000, Atta and al-Shehhi fly a small plane from Huffman Aviation to Miami International Airport and have a mechanical breakdown on a busy taxiway. They leave the airplane on the taxiway.

Jarrah arrives at Dubai Airport in the United Arab Emirates. Under questioning, Jarrah tells local authorities that for two months he has been in Pakistan, and -- they presumed -- in Afghanistan, in bin Laden's camps. Jarrah said he was a pilot, headed back to the United States. He had a valid U.S. visa, so Dubai let him go. Dubai authorities say they informed the U.S., but the CIA says the U.S. was not told about this trip until after Sept. 11.

In the months leading up to the attack, Atta makes several trips to Europe. U.S. intelligence believes he was mainly making contacts with the Al Qaeda network.

Atta flies from Miami to Madrid in January 2001 and returns to the U.S. six days later. It is not clear whom he meets, but Spanish police have recently rounded up a major Al Qaeda cell, which has direct links to Atta's Hamburg group.

When he returns to Miami, an alert immigration officer notices that Atta's tourist visa has expired, and that he has violated its terms by taking flight lessons. Atta is held for questioning. Finally, he is granted an eight-month extension.

That same month, the CIA learns that Khalid al-Midhar, who would eventually be identified as one of the hijackers aboard American Airlines Flight 77, which crashed into the Pentagon, was involved in planning the bombing of the USS Cole. His name isn't put on an immigration watchlist until the end of August, after he is already in the United States.

Atta travels to Prague, in the Czech Republic, where he was observed in a still unexplained meeting with an Iraqi intelligence agent.

Atta and his lieutenants move to the towns north of Miami. The second wave of the hijack team -- the "foot soldiers" -- begins to arrive from Saudi Arabia. They are conservative young Muslims who speak little English.

Atta receives a ticket in South Florida for driving without a license. He fails to show up for court and a bench warrant is issued for his arrest. He was never picked up.

Jarrah and Atta are both issued Florida driver's licenses on May 2, 2001.

Two Saudi members of the hijacking team fly to Newark, N.J., from abroad. They join the other Saudis who are renting an apartment above a convenience store in Patterson, N.J.

Atta travels to Spain. Twelve days later, he returns to the U.S. via Atlanta's Hartsfield International Airport. According to the indictment against Moussaoui, Atta purchases a knife in Zurich, Switzerland, on or about July 8, 2001.

Also according to the Moussaoui indictment, Jarrah travels from the U.S. to Germany on July 25 and returns on Aug. 4, 2001.

According to the indictment against Moussaoui, Jarrah takes a "check ride" at a flight school in Ft. Lauderdale, Fla., on Aug. 17. Five days later, he purchases an antenna for a global-positioning system (GPS), and schematics for Boeing 757 cockpit-instrument diagrams.

Al-Shehhi resides at the Panther Motel in Deerfield Beach, Fla. When he checks out, investigators now know, he was on his way to board United Airlines Flight 175, bound from Boston's Logan Airport to Los Angeles.

Moussaoui, a French Moroccan student, is jailed in Minnesota. He arouses suspicion as an inexperienced pilot seeking advanced jet simulator training for a 747 airliner. The flight school alerts the FBI, noting that a fully fueled 747 could be used as a bomb. The FBI now believes that Moussaoui was tapped to take bin al-Shibh's place as the 20th hijacker.

The three remaining members of Atta's Hamburg terrorist cell -- Said Bahaji, Ramzi bin al-Shibh, and Zakariya Essabar -- flee Germany, evidently headed for Afghanistan.

At Boston's Logan Airport, Atta watches and takes notes as American Airlines Flight 11 prepares for departure.

In the United Arab Emirates, a man named Mustafa Ahmed al Hawsawi collects cash sent back from the hijackers in America: $5,000 dollars each from Mohamed Atta and two other hijackers. According to Al Qaeda's rules, the hijackers must return any unused funds in order to finance future missions.

Atta and a young Saudi hijacker drive to Portland, Maine, where they check into the Comfort Inn. Various security cameras capture their images as they spend their final hours wandering around Portland.

Jarrah calls his girlfriend from the airport hotel in Newark. He is believed to be the pilot of United Airlines Flight 93, which originated in Newark and crashed in Pennsylvania.

In Portland, Maine, Atta and his Saudi companion catch a short flight to Boston's Logan Airport; they then board American Airlines Flight 11 to Los Angeles, seated in business class.

As Flight 11 is in line for takeoff from Logan, Atta calls al-Shehhi on his cellphone. Al-Shehhi is on the same tarmac aboard United Flight 175.