Their surprise was total, yet in hindsight they left clues everywhere. ANDREW LAXON and agencies follow the hijackers' trail from the Middle East through Germany to the heart of the United States.
Last Friday afternoon, one of the men believed to have reduced American might to rubble was sitting at a table in a Florida bar, drinking vodka and arguing about the size of his bill.
Four days later, with the September sun reflecting off the Hudson River, Mohammed Atta is thought to have been strapped into a cockpit seat on board the American Airlines Boeing 767 flown with such devastating effect into the first of the twin towers of the World Trade Center, killing thousands, including himself.
But last Friday, as he sat in Shuckhums Oyster Pub and Seafood Grill in Hollywood, 48km from Miami, throwing back Stolichnaya and orange juice, Atta's mind was fixed not solely on death.
Waitress Patricia Idrissi remembered that Atta had entered the bar at about 3 pm with his cousins, Marwan and Waleed Alshehri. Waleed wandered off to play a video machine at one end of the restaurant, while Atta and Marwan sat drinking and arguing between themselves.
Marwan Alshehri's drink of choice was Captain Morgan rum mixed with Coke. Over the next 90 minutes, both of these supposed Islamist extremists polished off five drinks each.
But as they were preparing to leave, Atta questioned his bill of $48 and Ms Idrissi called her manager, Tony Amos, to come over to try to help sort out the problem.
"I said to the guy, 'Hey, if you can't pay, let me know upfront and we'll work something out'," recalled the manager.
At this point, Atta got annoyed, pulled from his pocket a wad of $100 and $50 bills, paid his bill and left Ms Idrissi a nominal tip of just $3.
Mr Amos said: "He got angry and said 'You think I can't pay my bill? I'm a pilot for American Airlines. I can pay my f****** bill'."
In fact, 33-year-old Atta was a veteran Egyptian terrorist, who had been on a CIA-FBI-Immigration Service watch list since he was named as a suspect in a 1986 bus bombing in Israel. The FBI had him on a master list of suspected terrorists, specifically as a possible operative of the alleged mastermind behind the attacks, militant Muslim leader Osama bin Laden.
Marwan Alshehri, aged 23, was from the United Arab Emirates. He probably flew the second United Airlines plane which crashed into the second tower minutes after his colleague.
Waleed Alshehri's role remains unclear. He may have been part of the support crew for the 50-man terrorist attack, although one report suggests he was the suicide pilot at the helm of United Airlines Flight 93, which crashed into a field near Pittsburgh, killing all 45 on board.
The three are first known to have lived and worked together in Hamburg, where they spent several years studying German at the city's Technical University and plotting attacks on American targets with a group of Islamic extremists.
Last year they travelled to the United States for their flying training. From July to November Atta and Marwan Alshehri learned to fly light, single-engined Cessna and Piper planes at Huffman Aviation, a 25-year-old flying school in the small town of Venice on Florida's west coast, which takes 80 per cent of its students from overseas.
Calling themselves Afghanis, the two men paid the $10,000 fees for their lessons by cheque. Flying school owner Rudi Dekkers, former students and their ex-landlords Dru and Charles Voss, who asked them to leave after only a week, universally described them as anti-social, secretive and rude.
As his flying skills improved that year, Atta moved on to more advanced schools, first at the Embry-Riddle aeronautical university in Daytona Beach - where NBC reports that a fourth suicide pilot, Ali Mohammed al-Darmaki, trained before flying his suicide mission at the Pentagon - then at Lantana Airport near Palm Beach.
On December 29 and 30 Atta and another hijacker, thought to be Alshehri, received six hours of jet simulator training for a Boeing 727 at Henry George's SimCenter flight training school at Opa-locka Airport near Miami.
"I'm feeling terrible," said Mr George yesterday, noting that the two Middle Eastern men had not been very interested in takeoff or landing skills.
"They more or less concentrated in retrospect on doing turns and flying the airplane up in the air."
At some point Atta and Alshehri covered their tracks by stealing identification belonging to two Saudi pilots, Adnan Bukhari and his brother Ameer Abbas Bukhari, who lived in Vero Beach, halfway up Florida's eastern coast.
Pilot certificates in the names of both brothers and a photo of Adnan, which were found after the attack in a rental car used by the hijackers, later threw the FBI into confusion and led to an incorrect CNN report that the Bukhari brothers were two of the suicide pilots.
In the week before the attack five hijackers - probably including Atta, both Alshehri brothers and a fourth terrorist named as Ahmad Ibrahim Ali al-Hazoumi - hired a rental car and made up to five visits scoping out Boston's Logan Airport.
According to the Boston Globe, the men somehow obtained a pass allowing them to enter restricted areas.
The night before the suicide bombings at least two of the terrorists stayed at the Park Inn hotel chain in suburban Newton, Massachusetts.
On the morning of Tuesday they drove the rental car to an underground airport car park. Investigators would later find it contained flight manuals in Arabic.
Meanwhile, two other terrorists were joining their colleagues. They had slipped across the border from Canada on a ferry to Bar Harbour, Maine, even though they were on a border watch list because of their connections with an Islamic Jihad terrorist group.
At 6.45 am they flew in from Portland, Maine, but one of their bags disappeared at Boston airport. It was later found to contain a pilot training video, a copy of the Koran and a table for calculating fuel consumption rates on aircraft.
Five hijackers boarded each of the two Los Angeles-bound planes in Boston. In Washington and Newark, New Jersey, two teams of four were preparing to board the planes that would crash into the Pentagon and near Pittsburgh, respectively.
The terrorists had paid for their tickets by credit card. Atta used his real name, which amazingly did not trigger any alarm bells with airport security.
Atta made his way to seat 8D on American Airlines Flight 11, the first plane to hit the World Train Center. He was next to a man named Abdul Alomari.
Minutes later Marwan Alshehri got on board the second suicide plane, Flight 175.
America's nightmare was about to begin.
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