There are other key reasons why bin Laden's group, al-Qaeda, is the prime suspect in the bombings. First, the operation required recruits sufficiently well motivated that they were prepared to commit suicide. Al-Qaeda employed suicide bombers in the 1998 attacks against two US embassies in Africa and in the bombing of the USS Cole in Yemen 11 months ago.

Tuesday's attacks also needed the services of pilots capable of flying jets into their targets. Al-Qaeda has actively recruited pilots capable of flying such planes; in 1993 the group purchased a jet in Arizona, which was flown to bin Laden's then base in Sudan by a member of the group.

Al-Qaeda also has a long history of attacking American government buildings and US military targets, and bin Laden himself has recently indicated that he was planning to attack more American targets.

If indeed bin Laden is behind the worst act of terrorism in American history it would be the culmination of a crescendo of anti-American attacks undertaken by his network in the past decade, beginning with the 1992 bombings of two hotels in Aden housing US servicemen. That attack killed a tourist, but no Americans.

Within a year bin Laden's men were in Somalia, training Somalis how to hit US helicopters with rocket-propelled grenades, the method used against US Blackhawk helicopters in October 1993 during a two-day firefight in the Somali capital, Mogadishu, in which 18 US servicemen were killed. In 1995, four men influenced by bin Laden's writings, and trained in explosives in Afghanistan, bombed a military facility in Riyadh, Saudi Arabia, killing five US soldiers.

Three years later came the unprecedented bombings of the two American embassies in Africa, attacks that occurred within nine minutes of each other and killed more than 200 people. Al-Qaeda plotted a terrorism spectacular spanning the globe to mark the new millennium, planning to bomb the main Los Angeles airport, LAX; sink a US warship refuelling in the Middle East and bomb tourist sites and a hotel in Jordan. Those attacks, however, were, foiled either by police work or the incompetence of the plotters.

Last October bin Laden's followers bombed the USS Cole, one of the most advanced destroyers in the US Navy, as it was refuelling in Aden, an attack that blew a hole in the reinforced hull of the warship and killed 17 American sailors.

In past months there have been strong indications that bin Laden's followers were planning renewed anti-American attacks. In June the US embassy in Yemen was temporarily closed and FBI agents investigating the Cole attack were pulled out of the country as a result of threats emanating from bin Laden's followers.

That same month two men were picked up in New Delhi who said they were planning to blow up the visa section of the US embassy in the Indian capital, acting on the orders of the bin Laden lieutenant who had orchestrated the Cole attack. Then in July the State Department issued a warning that individuals might be planning terrorists imminent terrorist actions against US interests in the Arabian Peninsula.

Also this summer a skillfully edited two-hour-long recruitment tape made by al-Qaeda began circulating widely in the Middle East and appeared on the Internet. The videotape is not only a useful distillation of bin Laden's views, but also indicated that al-Qaeda was planning additional anti-American operations. This videotape is part of al-Qaeda's modus operandi, which is to subtly indicate a plot is in the works some months before it takes place, as happened in May, 1998, when bin Laden held a press conference in Afghanistan where he talked of "good news in coming weeks". Nine weeks later the US embassies were bombed.

A few months before the Cole bombing a videotape appeared in the Middle East that was notable for two things: bin Laden was wearing the curved jambiya dagger, typical of Yemen, which he had never previously worn in any pictures, and his deputy, Ayman al-Zawahiri, called for attacks on American forces in Yemen.

On the new videotape, bin Laden and his advisers make impassioned speeches about Muslims being attacked in Chechnya, Kashmir, Iraq, Israel, Lebanon, Indonesia and Egypt; speeches which are laid over graphic footage of Muslims being killed, beaten and imprisoned. For bin Laden, however, the greatest insult to Muslims is the continued presence of Americans in the holy land of Arabia. Bin Laden says: "These Americans brought women and Jewish women who can go anywhere in our holy land," adding that "the Arab rulers worship the God of the White House."

These statements are made over images of the Saudi royal family meeting American leaders such as the US Secretary of State, General Colin Powell.

On the tape bin Laden says that Muslims must seek revenge for these insults: "If you don't fight, you will be punished by God." The Saudi exile says the solution to the problems Muslims face is to travel to Afghanistan, and receive training about how to fight a holy war. The tape then shows hundreds of bin Laden's masked followers training at his al Farooq camp in eastern Afghanistan, holding up black flags and chanting in Arabic "fight evil". Chillingly, the tape also shows dozens of young boys, most of whom appear to be about 11, dressed in military camouflage uniforms, tackling the same obstacle courses seen earlier on the tape. On the tape bin Laden also makes the most explicit references he has ever made about al-Qaeda's role in a series of anti-American attacks, starting with Somalia in 1993: "Our brothers in Somalia told us the Americans were not brave." Then he discusses the 1995 bombing of the American base in Riyadh, saying: "We decided to get rid of this tyrant out of the holy land of Arabia. Then some of these youths responded to our call."

Next he explains that the 1998 bombing of the US embassy in Kenya was because it was "considered to be the biggest intelligence gathering centre in east Africa. With the help of God the hit against it was very strong."

Finally, he says of the Cole bombing: "Your brothers in Aden hit the Cole. When the collision happened it was the beginning of the war."

Towards the end of the tape, bin Laden implies more action against the United States: "The victory of Islam is coming. And the victory of Yemen will continue." Unfortunately, bin Laden appears to have made good on his threat.