Witnesses in at least two states have described the same person visiting the hijackers months before the attacks, in a pattern of behaviour that has been noted in previous attacks linked to Osama bin Laden. Officials described an older man, with salt-and-pepper hair, whose identity is as yet unknown.
The arrival of a senior leader to direct the final arrangements of an attack was a feature in the 1998 bombings of the US embassies in Tanzania and Kenya and unsuccessful plots in Jordan last year.
Clues to how the terrorists operated have been gleaned from hundreds of emails they sent to each other and from computer disks seized from homes where they were staying in Texas and Florida.
Flight bookings show that the hijackers conducted dummy runs on the flights they eventually hijacked to maximise their chances of success, investigators have revealed.
Reservations show they tried out several other routes. They chose early morning flights from Boston, Newark and Washington, investigators believe, because the planes were rarely full but would still have had a maximum fuel load.
The attention to detail has convinced investigators that a back-up team of up to 20 accomplices is still at large.
Also on the FBI's "watch list" are 11 people who shared addresses with at least one of the hijackers. At least seven are pilots or are believed to have had pilot training. Concern over a second wave of attacks has led the FBI to warn authorities in several cities, including Boston, Atlanta, and Richmond, Virginia, that they may have been a target.
The number of those detained in connection with the bombings reached 115 yesterday. Most have been held for violating immigration laws while at least six are likely to be charged as material witnesses, meaning they are suspected of having crucial information about the hijackings.
On Thursday, John Ashcroft, the attorney general, warned the acting governor of Massachusetts, Jane Swift, and Thomas Menino, the mayor of Boston, that the city could be targeted "within the next 48 hours". He said the threat was not based on firm evidence but on "raw information" that more strikes were planned for today.
Boston police added: "We are unaware of any specific, credible threats at this time."
The black box voice recorder on the United Airlines flight that crashed in Pennsylvania is being analysed, but the FBI's director, Robert Mueller, said it would take some time.
The black boxes from the planes that crashed into the World Trade Centre have not been recovered, and the black box from the American Airlines flight that hit the Pentagon was too badly damaged to be of any help
Anti-terrorist branch officers last night arrested three men and a woman in connection with the attacks on the World Trade Centre. One man, 27, and a woman, 25, were arrested in west London. A second man, 29, was arrested at a separate address in west London. The third man was arrested in Birmingham.