The indictment against Zacarias Moussaoui, the 33-year-old Frenchman accused of being part of the 11 September plot, provides the best evidence so far that London was an important base for al-Qa'ida's operations.
The 30-page document, charging Mr Moussaoui with six counts of conspiracy, underlines the suspect's nine-year stay in London and suggests Britain was among seven foreign outposts of al-Qa'ida activity. The others are: Malaysia, Kenya, Tanzania, Germany, Canada and America.
The information shows how a key member of al-Qa'ida's Hamburg cell, the prime organisational force behind the attacks on America, travelled to London in December 2000 on an apparent mission to recruit Mr Moussaoui and maintained contact with him for months.
What makes the indictment particularly valuable is that it is the first official document to reach the public domain in which Washington reveals its knowledge of the attacks and the way al-Qa'ida is believed to have planned them.
The most important "missing link" provided is the connection between Moroccan- born Mr Moussaoui and the Hamburg cell, all Gulf Arabs. The key person here, the indictment shows, was Ramsi bin Al-Shibh, a 29-year-old Yemeni who lived and worked in Hamburg with Mohamed Atta. Investigators believe he was supposed to join the suicide hijackings. He is missing and warrant is out for his arrest.
Mr bin Al-Shibh tried repeatedly to obtain a US visa, but was turned down. After his third failure he travelled to London for a week this time last year. As soon as Mr bin Al-Shibh left, Mr Moussaoui flew to Pakistan and did not return for three months.
The indictment does not spell out what Mr Moussaoui did there, but one strong suspicion must be that he went to Afghanistan either to train or to consult al-Qa'ida's leadership. The indictment also places him earlier at al-Qa'ida's Khaldan training camp "in or about April 1998".
Shortly after his return to London, Mr Moussaoui flew to America again, carrying $35,000 (£24,000) in cash, says the customs declaration he filled out in Chicago on 26 February. Most of that money was later deposited in a bank in Oklahoma, where Mr Moussaoui was enrolled in a flight school.
The contact with Mr bin Al-Shibh resumed at the end of July, in a flurry of phone calls between Oklahoma and Dusseldorf. At this time, Mr bin Al-Shibh received $15,000 by wire transfer from the United Arab Emirates and forwarded almost all of it by money order to Mr Moussaoui. One instalment was sent from Dusseldorf rail station and one from a rail station in Hamburg.
Investigators believe Mr Moussaoui was to be Mr bin Al-Shibh's replacement as the 20th member of the hijacking team. But the Frenchman aroused the suspicions of officials at a second flight school for commercial jets in Minnesota in mid-August and he was jailed, initially because of visa and passport irregularities.
After 11 September, he was transferred to New York as a material witness. Investigators found he had also researched crop-dusting, possibly to check ways of distributing chemical or biological agents from the air.
Mr Moussaoui's e-mail account was registered in Malaysia. In October 2000, around the time Mr Moussaoui was making applications to American flight schools, he had a letter from Infocus Tech based in Malaysia appointing him as a marketing consultant in the US, Britain and continental Europe.
The letter was found among his possessions when he was arrested. He also had two knives, binoculars, Boeing 747 flight manuals, a flight simulator computer program, fighting gloves and shin-guards, a hand-held aviation radio, a camcorder and a paper referring to a global positioning system receiver.
The indictment also names 33-year-old Saudi Mustafa Ahmed al-Sawhawi. He was given power of attorney over the United Arab Emirates bank account used to fund Mr bin Al-Shibh. Investigators say Mr al-Sawhawi left the UAE for Pakistan on 11 September. He, too, has gone missing.