In intelligence terms, these men were "sleepers" - agents who insert themselves into the local culture and lead apparently normal lives for years while awaiting orders.
Underneath their middle-class cover, some were keeping busy. When Kamfar wasn't munching pizza and scanning the Wal-Mart shelves, he apparently found time to visit the Philippines, home to the extremist Abu Sayyaf terrorist group, which has links with the chief suspect in last week's killings, Osama bin Laden. Fellow terrorist Saeed Alghamdi, who hijacked Flight 93 with Jarrah, went there at least 15 times as a tourist.
The alleged hijack ringleader and suicide pilot of Flight 11 from Boston to Los Angeles, Mohammed Atta - described by his father in Cairo last week as "decent, shy and tender" - quietly checked out Boston airport six months before the attack and may have been casing a US naval base.
And Khalid Almidhar, who steered American Airlines Flight 77 into the Pentagon, was filmed by the CIA in January last year at a meeting with a suspect in last October's bombing of the destroyer USS Cole in Yemen, believed to be the work of bin Laden.
Investigators confess there is still a good deal of guesswork in these attempts to reconstruct the terrorists' lives - for instance, an assumption that "John" Kamfar also used the name Abdul Alomari and that both may be false. The hijackers used a bewildering network of stolen identities, which has left the FBI admitting that its publicly released suspect list of 19 names may be full of errors.
A Saudi Telecom worker in Riyadh, who is threatening to sue CNN, claims he is the real Abdul Alomari and his passport was stolen in Denver, Colorado, five years ago. A former aircraft cleaner at Boston's Logan Airport says he is Saeed Alghamdi and his identity documents were stolen, while Ahmed Alshehri, a former Saudi diplomat in Washington and father of two of the alleged terrorists, reportedly claims one son, Waleed, is alive and living in Morocco, while the other, Wail, has been treated for mental illness.
As a result, the FBI is still struggling for firm leads in its biggest manhunt. Ten days after the hijackers killed more than 6000 innocent people, it has employed 4000 agents and 3000 analysts, received more than 100,000 tips from the public and detained 115 people.
Internationally, the hunt has been aided by an Interpol "September 11 Task Force" based at the agency's headquarters in Lyon, France.
On Monday, a grand jury is empanelled in New York to begin hearing evidence against any hijackers.
But so far the bureau's best breakthrough seems to be yesterday's news that agents have arrested Nabil Almarabh in Chicago.
The FBI has been looking for Almarabh since it discovered false identification documents, information about an American air base in Turkey and an unidentified airport runway plan at his address in Detroit, where three other men were arrested on Wednesday.
The former Boston man, who is being held on an assault charge, is suspected of helping the hijackers who destroyed the World Trade Center.
For the FBI, the stakes in this case are high. It has been knocked by a series of embarrassing bungles, from the incorrect naming of security guard Richard Jewell as a suspect in the Atlanta Olympic park bombing of 1996 to a failure to disclose documents which led to a delay of the execution of Oklahoma City bomber Timothy McVeigh.
Even as it tries to work out what happened last week, the FBI is engaged in a furious effort to stop any further attacks. Four thousand agents are working across America, trying to break up the network of terrorist cells that it is feared may still be operative in the US and planning further attacks.
One flight from San Antonio, Texas, to Denver was cancelled after investigators discovered five names from their suspect list of 200 were on board.
Last week's hijacks were led by 33-year-old Atta, who grew up in a middle-class family in Giza, on the outskirts of Cairo, where his father was a lawyer. He studied architecture at Cairo University, then moved to Germany and enrolled at Hamburg Technical University in 1993, where he studied engineering and town planning and shared a flat with Marwan Alshehhi.
Atta asked the university for a prayer room for himself and 20 other Muslim students, which is now thought to have been a cover for a terrorist cell planning attacks on US targets. Its members included three of the alleged suicide pilots - Atta and Alshehhi, who flew into the World Trade Center, and Jarrah, whose plane crashed near Pittsburgh.
Atta, who carried United Arab Emirates and Saudi passports, was a frequent visitor to America and lived in Daytona Beach, Florida, in the mid-1990s.
US intelligence sources say that before his final trip he met mid-ranking Iraqi intelligence officers in Europe, a claim denied by Iraq.
In May last year Atta arrived on a tourist visa with Alshehhi and lived in Florida, the hijackers' main base where many of them learned to fly. If records can be believed - and a trail of false IDs so far makes it hard to tell - his colleagues had been hard at work for years.
Hani Hanjour, who probably flew Flight 77 into the Pentagon, apparently racked up 250 flying hours from 1996 to 1999 to gain a commercial pilot's licence. Yet when he tried to rent a plane at Freeway Airport, Washington, in early August he performed so poorly in test flights that instructors would not let him fly alone. Despite this setback, Hanjour is believed to have flown a Cessna over Washington airspace three times in the weeks before the attack.
In Florida, Waleed Alshehri - thought to be Atta's fellow hijacker on Flight 11 - graduated in 1997 with a degree in aeronautical science and a commercial pilot's licence from the prestigious Embry-Riddle aeronautical university in Daytona Beach, where nearly a quarter of US commercial pilots train.
From last year the hijackers were all over Florida, flying, planning and, by most accounts, spending a lot of time at the gym.
Alomari, Mohald Alshehri and possibly Saeed Alghamdi trained at the FlightSafety academy in Vero Beach, half-way up Florida's east coast. Waleed Alshehri spent June and July at the Homing Inn motel in Boynton Beach with 25-year-old Satam al-Suqami, whose passport was later discovered in the rubble at the site of the World Trade Center.
Fayez Ahmed had learned to fly in Oklahoma but later moved to Delray Beach, near Palm Beach, where many of the terrorists were based.
Cousins Hamza and Ahmed Alghamdi lived with Ahmed Alnami in the Delray Racquet Club apartment complex. One resident said she thought they were drug dealers because they were coming and going at all hours, carrying dark bags.
From July to November Atta and Alshehhi learned to fly Cessnas at Huffman Aviation in the small town of Venice.
They then moved on to better schools, such as Embry-Riddle, to learn how to fly jets. In December they paid $1500 for six hours of jet simulator training for a Boeing 727 at Opa-locka Airport, near Miami.
On the West Coast, three of the Pentagon hijackers - Hanjour, Khalid Almihdar and Nawaf Alhamzi - lived in San Diego, where they flew Cessnas at Sorbi's Flying Club.
"I saw them watching and playing flight-simulator games when I was walking my dog at 10 or 11 at night," neighbour Ed Murray told Time magazine.
Almihdar and Alhamzi may have come closest to getting caught. The CIA, which had filmed Almihdar with the USS Cole suspect in Kuala Lumpur, tipped off the FBI that the two men had been spotted coming in and out of the country.
They returned in July, listing a Marriott hotel in New York as their address. But it took two weeks for the FBI's New York office to pass the men's names to Los Angeles and the San Diego office was not informed until even later.
Only after the attack did the bureau track down the men's address.
On September 17, four days before they faced their death, the hijack leaders in Florida overcame their Islamic fundamentalist principles and went on a bender at Shuckums Oyster Pub and Seafood Grill, Hollywood, near Miami, where they had a row with the manager over the $48 bill. A "wasted" Atta insisted he could afford to pay with the now chilling words: "I'm a pilot for American Airlines."
In the days leading up to the attack the hijackers seem to have become less cautious. A huge paper trail shows how they booked their flights and rented cars and hotel rooms, giving the FBI increased hope of making progress.
Security camera footage has established that Atta and Alomari entered through Canada, then flew from Portland to Boston on the morning of September 11. They were spotted rushing to make the plane at 5.45 am.
Another lead for the FBI could be the regular visits by Saeed and Ahmed Alghamdi to the Philippines. Ahmed left Manila on September 10, the day before the hijackings.
Investigators are particularly interested in $10,000 which the hijackers sent to one man in the United Arab Emirates.
Atta sent two Western Union wire transfers on September 8 and 9. Marwan Alshehhi and Waleed Alshehri sent their money just before getting on board the hijacked flights to Boston.
The FBI believes the money used for the attacks comes from one source and tracking it could be the key.
However, it expects the trail will eventually become bogged down in an ancient system of banking called hawala, meaning "word of mouth", alleged to be used by bin Laden.
One disturbing pattern has already emerged in the investigation. These terrorists were not like the poverty-stricken, rock throwing teenagers of the West Bank and Gaza strip, who grow up to be suicide bombers against Israel, fuelled with images of martyrdom and the rewards which await them in heaven.
They were well-educated, middle-class citizens of the world, who had families, drank alcohol and moved comfortably between Europe and America.
In the end they used their Visa cards, frequent-flyer numbers and the internet to buy first-class tickets to their deaths - a trend which has analysts deeply worried.
"People who have a lot of other reasons to live for are deciding this is such an important cause that they are willing to die anyway," said terrorism expert Andrea Talentino.
"That, obviously, is very frightening."
Map: Opposing forces in the war against terror
Afghanistan facts and links
For coverage of the attacks on the United States, see:
Full coverage: Terror in America
