They seemed like the kind of young men, one of their landlords said later, that you'd want to take to a baseball game.

One of them liked Stoli vodka and orange juice. Three of them liked to splash in their apartment block's pool, grown men playing together like children. A few were married, with small children who played with the American kids in their neighborhoods.

Another advertised for a Mexican bride because, he said, he had heard they made good wives. Trouble was he couldn't speak Spanish, so someone taught him a few phrases - things like "Que pasa?" (what's happening?)

Another (their leader, some say) was being sought - by a collection agency for failing to return three rented videos, Ace Ventura, Vampire and Storm of the Century Part II. His only other trouble had been for driving without a licence.

He was smart, technically proficient and well educated, this Mohamed Atta. He'd received a degree in urban planning at the Technical University in Hamburg, Germany. In a terrible irony, given what was to follow, his thesis was on urban renewal - how to improve a city.

The 19 men the FBI has identified as suspects in the attacks on the World Trade Centre and Pentagon last week did not outwardly fit the Hollywood image of Middle Eastern terrorists. They were young, anonymous, living in nondescript suburban homes and apartments across the US - in Florida, Massachusetts, New Jersey, Arizona and southern California.

Theirs was a subtle presence in the US for many months, in some cases years. They didn't blend in exactly, but they stayed out of trouble. In spy parlance, they hid in plain sight.

And, as The Washington Post pointed out on Saturday, these conspirators didn't slip into the country in the dark of night. They were welcomed in daylight. Some, if not all, arrived legally, with visas. One graduated from a flight school in Daytona Beach, Florida. Two others may have received schooling from the American military itself in exchange programs at the Air War College at the Maxwell Air Force Base in Alabama and the Defence Language School at Lackland Air Force Base in Texas.

They tended to pay for things with wads of cash. At times they could be stand-offish, and made 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 at night?

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".

In the past few weeks, they began disappearing from their neighborhoods. They hid in low-rent motels, leaving behind in one instance wastepaper baskets filled with Boeing flight manuals and aeronautical maps. In late August, over three days, they booked first and business-class airline tickets, paying for them with credit cards and even registering for frequent-flyer points.

Then they apparently reappeared on Tuesday morning, in the deadliest terrorist attack in US history.

One of the questions that has been gnawing at Americans since has been how the nation and its authorities failed to detect the massive conspiracy festering in their midst. Too late, the 19 names have appeared on the FBI's list of suspects, but even that is smudged with uncertainty. The agency said it did not know for certain where most of the men came from, exactly where they lived in the US or how old they were. Several had names so common in the Middle East that tracking them down might never be possible - common Saudi tribal names like minor variations on a theme: Alhamzi, Al Suqami, Alghamdi, Alshehri, Alhaznawi, Alnami, Alomari. If they used false names when they boarded their flights last Tuesday morning, it might be even more difficult to ever discover their true identities.

Despite the pains they took, they left some tantalising clues behind, including credit card charges for $US50,000 worth of air tickets and, in a car left behind at Boston's Logan Airport, a suicide note in the name of Mohamed Atta.

Atta, 33 and apparently the oldest of the conspirators, was born in Egypt, went to university in Hamburg and most recently had been living in Coral Springs, near Fort Lauderdale, Florida. He is believed to have piloted American Airlines flight 11, the first to crash into the World Trade Centre.

His letter, found in his luggage at Logan with his passport, international driver's licence and instructional videos for flying Boeing airliners, said he planned to kill himself so he could go to heaven as a martyr. But some reports have said the letter was dated 1996, suggesting the operation was years in the planning. In 1996 Atta was in Hamburg, believed to be a major European centre of operations for followers of fugitive Saudi terrorist suspect Osama bin Laden.

His thesis adviser there, Dittmar Machule, described Atta as "a very nice young man: polite, very religious and with a highly developed critical faculty".

At the time 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 Atta and Al-Shehhi had organised a terrorist cell in the city "with the aim of launching spectacular attacks on the institutions of the United States". Neighbors say the men hosted meetings late at night. Another man who lived in their apartment, they say, was Waleed M. Alshehri - identified by investigators as being on flight 11.

On Saturday Nehm said Ziad Jarrahi, another man identified by the FBI as having been on United Airlines flight 93, which crashed in Pennsylvania, had lived in the same apartments. Jarrahi had lived there for four years, studying aircraft construction, before moving to the US in June 2000.

Jarrahi, a Sunni Muslim, was from the village of al-Marj in Lebanon. German investigators said police found "aeroplane-related documents" at the home of his girlfriend. His father, Samir, said he had received news that his son died on one of the planes, but was not convinced he was a hijacker. An uncle, Nazrem Jarrah, said: "He didn't pray or fast. He never cared about politics or organisation. All he cared about is having fun and drinking beer. If he was actually on the plane, then he is one of the passengers, not a hijacker."

Mohamed Atta obtained a visa at the US consulate in Berlin in May 2000 and went to Newark, New Jersey, on June 3 on a flight from Prague, Czech Republic, under a temporary visitor's visa good for six months. He overstayed his visa by more than 30 days. Then he took several trips to nations with terrorist cells where face-to-face meetings could frustrate electronic surveillance. On January 4, 2001, he flew to Spain from Miami.

Nearly a week later he returned. Despite previously overstaying his visa, Atta was readmitted for another six months. There, he and Al-Shehhi diligently pursued flight lessons, first at Huffman Aviation in Venice, Florida, then at SimCenter Inc, training on a Boeing 727 full-motion flight simulator.

On July 7, two months before the attack on the World Trade Centre and the Pentagon, Atta again flew to Spain. He returned on July 19. On August 6 he started leasing vehicles from Warrick's, a Florida car rental agency. Owner Brad Warrick said his company had rented cars to Atta three times. Over the next month he drove the cars more than 4800 kilometres.

Warrick was watching the news on Wednesday when he saw authorities towing a car at Boston's Logan International Airport. "I saw the picture on the screen and said, `That guy looks familiar'," Warrick said.

He had Atta's rental contract faxed to him and called the FBI. The car had not been touched since it was returned by the suspects because it was due for an inspection and could not be rented again. It was a clean, perfect item for the FBI to lift prints from.

"He (Atta) just seemed like a businessman - everything about him, his demeanor, the way he looked," Warrick said. "He would wear nice slacks and a polo shirt. He was articulate, spoke English very well. He seemed like he had been in the country for some time. He just seemed like an everyday, local guy."

Across the country, other hijackers kept a similarly low profile. In 1999 Nawaq Alhamzi had moved into the Parkwood Apartments in San Diego, California. Manager Holly Ratchford remembers him as polite and an "attractive guy", clean-shaven, about 162 centimetres, with a thin build. He paid his rent on time and never caused trouble.

He spent almost all his time with three other men, said neighbor Freddy Evans. They used the pool, but only when no one else was there, he said. "They were strange. Three grown men playing in the pool like kids."

Alhamzi and Khalid Al-Midhar moved into a room in Lemon Grove, a quiet neighborhood just east of San Diego, renting rooms from Abdussattar Shaikh, a retired English professor at San Diego State University and co-founder of San Diego's Islamic Centre.

Alhamzi told Shaikh he had gone to the US to learn English, though he rarely seemed to attend classes. "He was a loner and he didn't talk much," Shaikh said. "I don't think he had any friends. While he lived with me, I never saw him use a telephone. I wondered if he had any family at all."

But he wanted one, he said. It was Alhamzi who said he wanted to marry a Mexican girl.

Neighbor Denise Adair said the two men had "seemed like nice, normal people. You never imagine that you have a hijacker living next door".

On Tuesday, the FBI says, the two friends were on board American Airlines flight 77 when it crashed into the Pentagon.

Hani Hanjour, believed to have been the pilot on that plane, appears to have lived in Arizona for the past five years and received pilot training at CRM Airline Training Centre in Scottsdale, Arizona. 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, company official Gerald Chilton said, "never completed the course. He was not believed proficient enough to obtain a licence. He just wasn't a good student with the dedication we see in US Air Force pilots that train there or European airline pilots. Not that he was rude or impolite. He was just described as a difficult student".

In all, 12 of the 19 suspects named were said by the FBI to have lived at one time in Florida, though in three separate locations. Members of the groups in each of the locations appear to have been separately responsible for individual hijackings. This is in keeping with the common view among investigators that Islamic terrorists typically operate in individual cells, largely uninformed of other cells.

US officials said all the suspects appeared to have entered the country legally. An emerging view in the intelligence community was that the hijackers had not arrived with the plan intact but were recruited later or came as terrorist sympathisers pledged to help when asked.

It is in Florida, where several of the men received flight training, that the agency has gathered many of its best clues.

At the Panther Motel in Deerfield Beach, they found an apparent treasure trove. When owner Richard Surma took out the rubbish on Monday morning, he discovered a stack of flight manuals for Boeing 757s, detailed aeronautical maps of eastern US states, a flight-school tote bag and a protractor. There were also illustrated books on karate and jujitsu and a box-cutter knife. When police canvassed the neighborhood after Tuesday's hijackings, Surma called an officer to tell him what he had found.

On August 26, Marwan Al-Shehhi and another man had checked into room 12, paying $500 in advance. Surma noticed that the men didn't go to the beach, but rather preferred to spend their time around the motel's small swimming pool. A third man visited often. The impression they made was, as always, fairly innocuous.

"They were very neat and very polite," Richard Surma said.

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, Florida, 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."

Thirty-five kilometres north in Boynton Beach, agents searched a room at the Homing Inn where a Waleed M. Alshehri had stayed. Alshehri, identified by the FBI as one of the hijackers on American Airlines flight 11, spent a month and four days at the hotel, said the owner, who requested anonymity. "Every time he came to pay, it was `Hi, how are you?' He was a normal guy. When he checked out, the room was clean and neat."

Alshehri's driver's licence listed a permanent address at a seaside motel in Hollywood, Florida. Joanne Solic, one of its owners, said one of the suspects shown in an FBI photo had stayed at her hotel with another Arab man all of May and a few days in April. "They were nice kids - clean-cut, nice looking and courteous," she said.

Many details of the nice kids' plan remain unknown, but its execution began no later than August 25, when the hijackers began buying plane tickets. In many cases they used Internet travel agencies. Money was apparently no object. Two of the men paid $4500 each for one-way first-class tickets on United Airlines flight 175 - putting them close to the cockpit.

Why they picked September 11 is unknown. Possibly they selected it because it was a Tuesday, a light day for cross-country travel. Fewer passengers would mean easier crowd control. The technology-savvy terrorists could easily have shopped for uncrowded flights by examining airline websites.

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 that Atta and Alomari rented a car in Boston, drove to Portland, Maine, and took a room on Monday night at the Comfort Inn south of town. They then flew on a short flight on 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 5.44am - 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 on Tuesday. "They were joined at the hip," said Quirion. The two men struck him as clean-cut, wearing slacks, dress shoes, long-sleeved shirts and carrying dark shoulder bags. Their hair was closely cropped. They had no facial hair. In short, they looked like typical businessmen. Unmenacing.

It all shows how wrong you can be.

But so too can be the most painstaking investigation.

Already there has been confusion. US newspapers told of one suspect, Abdul Rahmen Alomari, a flight engineer, living in Florida's Vero Beach with his wife and four small children. The adult Alomaris didn't socialise much, they reported. They spent time with another Muslim family - clannish behavior that the neighbors assumed was normal. But they had "late-night meetings".

Still, when Alomari told his landlord in August that the family would soon be moving back to Saudi Arabia, he did something unusual, throwing a farewell party for all the neighborhood kids. "They invited all the kids, even ones they'd never seen before," neighbor Andrew Krease said. Where they came from, Mrs Alomari explained, it was customary to throw a party before moving - to leave nice memories.

At 5.30am on Wednesday, barely 20 hours after the attacks, 60 FBI agents and local law enforcement officers swept into the Alomaris' former neighborhood. Fearing that the white stucco house they had rented might be booby-trapped with a bomb, they ushered neighbors down the street for their protection.

They searched the house for hours, as well as a neighboring house that had been rented by Adnan Bukhari, another pilot trainee from Saudi Arabia and a friend of Alomari. Bukhari consented to the search of his house, then agreed to be flown to Miami for a lengthy interview at the FBI's office, his lawyer said. Bukhari took and passed a lie detector test on Thursday and was allowed to return home that night.

During the interview, Bukhari's mobile phone rang. Bukhari answered, then asked the FBI agent whether he wished to speak to the caller. The agent found Abdul Rahmen Alomari on the line, calling from Saudi Arabia, where he had always said he would be. He was calling after hearing news reports that he was considered a suspect and wanted to clear his name.

The man the FBI wanted was Abdulaziz Alomari.

- with AGENCIES