The FBI believes it has identified 19 hijackers, all ticketed passengers, involved in the seizure of the four planes on Tuesday - five on three of the planes and four on the remaining one. All are said to have been linked in one way or another to the Saudi millionaire, Osama bin Laden.
In an investigation involving 4,000 FBI agents and 3,000 ancillary staff across the country, police and FBI have carried out raids from Boston to Florida.
They have built an extraordinarily detailed picture of a tight-knit group who used the cover of their families to live in suburban communities. Their children attended local schools while they learnt to fly jets at US air schools.
Some of the suspects are believed to have sent their families back to the Middle East ahead of the attacks and even purchased furniture to be shipped home.
Early yesterday, investigators recovered the voice and data recorders from the jet that slammed into the Pentagon. On Thursday, searchers found the flight data recorder from the hijacked plane that went down in Pennsylvania.
Struggling to get back to normal operations, US airlines have received a list from the FBI of 52 people, most of Middle Eastern descent, whom authorities want detained if they appear at airports. Several people across the country were being questioned or held on immigration charges.
Meanwhile, fears that police had thwarted another two attempted hijackings at New York's La Guardia and Kennedy airports on Thursday were allayed yesterday and seven people arrested overnight were released.
Ultra-cautious staff had been alarmed by a Saudi pilot travelling on his brother's passport, and by a group of four whom staff believed tried to board a flight on Tuesday.
Police stormed a plane at Kennedy and took off two women and a man, while at La Guardia the four were questioned. All flights were grounded and both airports closed.
In Vero Beach, Florida, police discovered the temporary home of a Saudi pilot, Abdul al Omari, next door to a friend, Adnan Bukhari. The pair had large families who returned to the Middle East a few days before the attack.
Both men were training at the local Flight Safety Academy. Mr Omari is believed to have been involved in seizing and then crashing one of the two Boston planes.
Mr Bukhari is being questioned by police and is reported to be "co-operative".
A regular visitor to their homes, Amer Kamfar, who once used Mr Bukhari as a reference, is on the run and is being sought by the police. His neighbours say he lived quietly a couple of miles away with his wife and four children.
Further up the coast in Daytona Beach, police are investigating another student pilot, Mr Waleed al Shehri, who died on one of the planes.
He studied at the Embry Riddle Aeronautical University for four years, qualifying as a commercial pilot in 1997 and leaving the area a year later. Also Saudi, his fees were paid by the Saudi government, which subsidises huge numbers of its citizens to study abroad.
"A very mild-mannered person, small in stature," a teacher at the college, Dr Frank Richey, told the New York Times. "He seemed to be very friendly. He was probably one of the last people I'd expect to do something like this. He didn't appear to be a religious fanatic at all."
The college has a substantial foreign enrolment from all over the world.
Another suspect, Mohemed Atta, identified from TV monitors boarding one of the Boston flights, apparently lived in neighbouring Port Orange at the same time. Police have searched a flat he leased between May and June in Hollywood, Florida, with Marwann Alshehhi, who died on the other Boston flight.
Both men's movements have been traced to a Florida flying school and back to Europe, where they spent time in Hamburg.
Mr Atta studied for a degree in construction engineering in Hamburg. In Germany, both men are remembered as Islamic militants.
For three days last month, Mr Atta hired a single-engined plane from yet another school at $88 an hour "to increase his flying hours", although staff noted that he already had a commercial pilot's licence. CNN reports that he earlier bought flight simulator time at another centre at $1,500 an hour. Trainers recall that he and two companions were most interested in level flight and steering. Their suspicions were raised by the group's lack of interest in landing procedures.
A car found at Logan Airport on the day of the attacks has been linked to Mr Atta. It contained a manual in Arabic on flying 767s and a copy of the Koran.
Investigators have also searched a house in Vienna, Virginia, where several of the suspects are believed to have stayed for some time.
Evidence that the suspects used numerous schools to learn and practise their flying has also led to a scouring of school enrolment lists to identify future potential terrorists. Miami alone has four such schools, training hundreds every year.
In Minnesota, the possibility emerged that the FBI knew before Tuesday's attack of at least one Arab man seeking the type of flight training the hijackers received.
US officials confirmed that a few weeks ago the FBI detained an Arab man in Minnesota when he tried to seek flight simulator training for a large jetliner. Officials said the FBI had no reason to charge him at the time and instead began deportation proceedings. He is being held but is not co-operating with the FBI.
US and Philippine authorities searched a Manila hotel in connection with the investigation. Philippine officials also questioned a Saudi Airlines pilot and refused entry to nine Malaysian men suspected of having undergone terrorist training.
Mexico's Defence Secretary, Gen Rafael Macedo, said authorities were searching the country for at least nine people who may have helped to plan the attacks.
The Canadian authorities denied suggestions there was evidence that the suspects had slipped across the largely open border between Canada and the US. American police have said they believe two may have slipped across at a small town crossing in Maine.
Intelligence officials have told the New York Times that in August the CIA intercepted messages to the Saudi-based family of bin Laden, warning that they should leave the country "before the work is done". They say that the CIA at that time also issued a report, warning in unspecific terms of bin Laden's desire to shift his focus to targets in the US.
The full list of the names of the suspected hijackers was released by the US Department of Justice last night.
They were: Marwan Al Shehhi, Fayez Ahmed, Mohald Alshehri, Hamza Alghamdi and Ahmed Alghamdi, who were on United Airlines Flight 175, which destroyed the World Trade Centre's twin towers.
Waleed M. Alshehri, Wail Alshehri, Mohamed Atta, Abdulaziz Alomari and Satam Al Suqami, were aboard American Airlines Flight 11, which also destroyed the World Trade Centre.
Khalid Al-Midhar, Majed Moqed, Nawaq Alhamzi, Salem Alhamzi and Hani Hanjour were on American Airlines Flight 77, which crashed into the Pentagon building.
Ahmed Alhaznawi, Ahmed Alnami, Ziad Jarrahi and Saeed Alghamdi were on United Airlines Flight 93, which crashed in the Pennsylvania countryside.
