Khalid Sheikh Mohammed was arrested in March
The original plans for the 11 September terror attacks envisaged as many as 10 planes, with additional targets on the American West Coast and in Asia, according to a key al-Qaeda suspect now in US custody.
Khalid Sheikh Mohammed has told his American interrogators that Osama Bin Laden started planning the attacks in 1996, the Associated Press reports.
The interrogation records seen by AP indicate that al-Qaeda was still planning attacks on the US, Israeli and other Western targets this year.
Khalid Sheikh Mohammed was detained in Pakistan in March, and is now being held by the CIA at an undisclosed location.
As well as the 11 September attacks, Sheikh Mohammed was also involved in working on a possible attack on Singapore, and a plan to blow up 12 Western aircraft simultaneously over Asia.
The interrogation details seen by AP suggest that this plan evolved into the four 9/11 hijackings.
Before the attacks on New York and Washington, Sheikh Mohammed said he started working with Jemaah Islamiah (JI), the group suspected of being behind the Bali bombing in October 2002.
Khalid Mohammed starting "recruiting JI operatives for
inclusion in the hijacking plot as part of his second wave
of hijacking attacks to occur after 11 September," according to material obtained by AP.
Khalid Mohammed says he went to Osama Bin Laden in 1996 to convince him "to give him money and operatives so he
could hijack 10 planes in the United States and fly them
The original plan was to pick five targets on the East Coast, and five on the West Coast, but Bin Laden did not believe that was practical, the reports state.
Initially, Osama Bin Laden offered Khalid Mohammed four men - two Saudis and two Yemenis.
The two Saudis - Khalid al-Mihdhar and Nawaf al-Hazmi - were on the plane that was flown into the Pentagon.
But the two Yemenis - Walid Muhammed Bin Attash and Abu Bara al-Yemeni - were unable to obtain US visas, and were diverted to work on the Asian hijack plans.
The version of events given in the interrogation reports suggests that al-Mihdhar and al-Hazmi were at the centre of the 9/11 plot, rather than Mohammed Atta, who was initially identified by US as the ringleader.
Khalid Mohammed said that originally the hijackers were to have come from many different countries, but that Osama Bin Laden decided to use mainly Saudis.
US investigators have suggested that Saudis were chosen partly because they could easily obtain US visas.
However, Khalid Mohammed told investigators that he knew nothing about Omar al-Bayoumi, a Saudi who helped the pair when they first arrived in California.
This casts further doubt on a conjecture in a US Congressional report that Mr Bayoumi was a Saudi intelligence agent.
The Saudi Government strongly denies this, and FBI investigators were also sceptical after several interviews with Mr Bayoumi.
The original four plotters were sent to Afghanistan in 1999, for training at one of Bin Laden's camps.
There they were taught mainly commando skills, rather than how to pilot jet planes.
Operatives were also instructed on how to blend into American society, research airline timetables and use telephone books.
Later, other operatives were trained in Malaysia by agents of Riduan Isamuddin Hambali, JI's alleged operations chief.
Among those trained there was Zacarias Moussaoui, a Frenchman now in detention in the US, on suspicion on conspiracy in the 11 September attacks.