The congressional commission investigating the 11 September 2001 attacks on the US has detailed the complex planning involved.
The following are key points from the preliminary report - Staff Statement No 16, Outline of the 9/11 Plot - published on 16 June 2004.
The commission's final report is expected on 28 July.
Sheikh Mohammed's idea
The report is based on reported statements by 9/11 conspirators and materials supplied by the FBI, CIA etc on interrogations of captured al-Qaeda members.
The idea for the attacks appears to have originated with Khalid Sheikh Mohammed (KSM), whose nephew Ramzi Yousef masterminded the 1993 World Trade Center bombing.
KSM met Osama Bin Laden in Afghanistan in the spring of 1999 to select targets: the White House, the Pentagon, the Capitol and the World Trade Center.
KSM trained the first two suicide operatives recruited - Saudi nationals Nawaf Alhazmi and Khalid Almihdhar - basic English, and how to read a phone book, make travel reservations, use the internet and encode communications.
They also used flight simulator computer games and analysed airline schedules to figure out flights which would be in the air at the same time.
An al-Qaeda operative made a test flight from Kuala Lumpur to Hong Kong, aboard a US airliner, managing to smuggle aboard a box-cutter concealed in his toiletries bag.
Pilots and muscle
Alhazmi and Almihdhar arrived in Los Angeles on 15 January 2000 - the first 9/11 operatives to enter the US - but had little luck in learning English or taking flight lessons.
Four Western-educated Islamists living in Hamburg were ideally placed to become suicide attackers: Mohamed Atta, Marwan Al-Shehhi, Ziad Samir Jarrah and Ramzi Binalshibh.
Binalshibh, a Yemeni, could not get a visa for the US and he became a key witness after his eventual arrest.
The other three, all Saudis, arrived in the US in May-June 2000 where they soon began flight training and went on to become 9/11 pilots.
The fourth 9/11 pilot, Saudi national Hani Hanjour, arrived in the US in December 2000 where he brushed up on his existing flying skills.
Having selected four pilots, al-Qaeda chose 15 "muscle hijackers": Alhazmi, Almihdhar and 12 other Saudis, plus one UAE national.
The new "muscles" began arriving in the US in late April 2001.
Summer of training
The pilots spent the summer of 2001 with cross-country surveillance flights and additional flight training in the US, managing to smuggle box-cutters aboard. The "muscles" trained in gyms. Knives were probably bought in.
They determined that the best time to storm a cockpit would be about 10-15 minutes after takeoff, when cockpit doors were typically opened for the first time.
There were plans to crash the planes into the ground if they could not strike their targets.
Just over two weeks before the attacks, the conspirators purchased their flight tickets.
Finally the hijack teams arrived at their chosen airports, the overall operation coordinated by Atta.
The cost of the operation was between $400,000 and $500,000.
According to KSM, al-Qaeda intended to use 25 or 26 hijackers for the 9/11 plot, as opposed to the 19 who actually participated.
KSM seems to have wanted to replace one of the pilots, Jarrah, with Zacarias Moussaoui, now in US custody.
Jarrah apparently had personality clashes with Atta during the planning and was viewed as a security risk for a time.
Although Atta enjoyed wide discretion as tactical commander, Bin Laden had strong opinions regarding both the date and the targets.
Bin Laden first requested a date of 12 May 2001.
KSM reports that al-Qaeda's Taleban allies in Afghanistan opposed attacks on the US under pressure from Pakistan.
Bin Laden argued the attacks needed to be carried out immediately to support the Palestinian uprising and protest at the presence of US military forces in Saudi Arabia.
Bin Laden also thought that an attack against the US would reap al-Qaeda a recruiting and fundraising bonanza.