US investigators and intelligence agencies turned the weight of their inquiry towards Osama bin Laden, the Saudi dissident, with government officials saying they had gathered increasing evidence that pointed to the Afghanistan-based fugitive.

While experts familiar with Middle East terrorism warned that there was still the possibility of state involvement -possibly Iraqi - the US government inquiry, a joint effort by the Federal Bureau of Investigation and the Central Intelligence Agency, appears to be focusing on Mr bin Laden's terrorist network, al-Qaeda ('the Base').

US investigators said the flight manifests of one of the hijacked aeroplanes listed the name of a suspected bin Laden associate, and officials briefed by US intelligence organisations said counter-terrorism agents intercepted two phone calls between people associated with al-Qaeda after the attack, discussing successful strikes on US targets.

Through Arab-language newspapers based in the Middle East, Mr bin Laden denied involvement and sought to distance himself from the attack.

Senior US officials were careful not to name Mr bin Laden as the probable perpetrator but they said the government investigation had gathered a substantial amount of data pointing them in one direction.

"There is evidence being developed now, and good evidence," said Colin Powell, US secretary of state. "We will be able to make a definitive statement in due course."

Daniel Benjamin, at the Centre for Strategic and International Studies, said: "The ultimate hallmark of the bin Laden modus operandi is the massive carnage. There is no other terrorist on this earth who seeks this kind of bloodshed."

Born in 1957 and raised in Jeddah, a steamy coastal town on the shores of the Red Sea, Osama bin Laden lived a privileged life as the son of one of the most preeminent and wealthiest businessmen in Saudi Arabia. He and his more than 50 siblings were sheltered from the poverty of Saudi Arabia prior to the oil boom, but did not escape the severe traditions of Wahhabi Islam.

Under those customs, boys were brought up by male tutors, learning the Koran by heart (as many still do) and by the age of 9 were taken from their mothers to be raised by their fathers.

Osama's father, Mohammed Awad bin Laden, emigrated from southern Yemen's Hadramawt valley to Saudi Arabia in the 1920s and became a close friend of that country's first monarch, Abdul-Aziz Ibn Saud, whose example he followed by taking almost as many wives as the King. He fathered so many children in part to consolidate his strategic alliances through extended families. (Osama would later take a page out of that same book by marrying one of his daughters off to the Taliban leader Mullah Mohammed Omar.)

Thanks in part to the family's relationship with the King, the Bin Laden Organisation, the family's construction company, prospered. The group - which was founded in 1935 and became synonymous with Saudi Arabia's oil-fed development - has an annual turnover in the billions of dollars and employs some 35,000 people with offices throughout the world, including the Americas.

Its first headline-grabbing success came in 1961 - shortly before Mohammed bin Laden died in an aeroplane accident in the US - when it built the Mecca-Taif mountain road in Saudi Arabia's Western province. The group was awarded successive contracts to expand the shrines at Mecca and Medina (the two most sacred Islamic sites) and more recently completed the 1,000ft Faisiliya Centre in Riyadh and the new Kuala Lumpur international airport.

For all their fame, the family has remained intensely private, and visitors, particularly journalists, are cautioned not to bring Osama bin Laden's name into any conversation. But it is hardly surprising, say those who have known the bin Ladens, that in such a large family at least one black sheep should emerge.

Though Osama is said to have studied management and economics in Jeddah, he was better known in his 20s as a playboy and party-goer in Beirut, which he visited regularly.

However, Beirut lost much of its attraction for playboys after civil war broke out in the mid-1970s, which may have had as much to do with Osama's "conversion" as his increased awareness of the desperation of the "poor and dispossessed" among radical Palestinian groups.

The genesis of Mr bin Laden's conversion took place around 1978 - before Ayatollah Khomeini started to propagate the Islamic revolution in Iran. But Mr bin Laden's spiritual conversion did not take hold until the mid-1980s when he met Mohammed Ayman Zawahir, the Egyptian Jihad party leader, in Peshawar, on the Afghan border.

In February 1979 the Islamic revolution in Iran prompted the 22-year-old Mr bin Laden to yearn for a similar revolution in the Arab world, according to Hamoud Salih, professor at California State University, Dominguez Hills.

Instead two disasters followed in swift succession. In November 1979, the grand mosque in Mecca was overrun and the following month the Soviet Union invaded Afghanistan. The conflict in Afghanistan provided Mr bin Laden with an opportunity to do something for the cause, even if it meant accepting financial and logistical help from, not only the Saudi royal family government which he held in contempt, but also from the US and the CIA.

For the next decade Mr bin Laden worked and fought under the approving eye of the US and Saudi establishments that he inwardly despised, to evict the Soviets from muslim territory.

Entering the Afghan campaign, Mr bin Laden's lack of political background was offset by courage and energy and deep commitment to a more permanent cause. Still, he witnessed only a few engagements. Most of his early years in Afghanistan were spent in trying to gauge the relative strength of the various factions, so that he would know which one to ally himself with.

His most important asset was money. According to US and Gulf Arab analysts, US and Saudi agencies, as well as other Arab governments which supported the Afghanis out of domestic concerns, channeled hundreds of millions of dollars into supplying the guerillas, both Afghani, and Arab-Afghanis like Mr bin Laden, who hardly needed to use his own inherited fortune, estimated today at between $250m-$300m.

With US and Saudi funding, he recruited thousands of followers from Islamic communities around the globe, (by one count 4,000 followers from Medina alone).

Their revolutionary zeal had been refined and enhanced by the time the Soviet soldiers withdrew but their own governments, fearing anew their potential for destabilising their regimes, turned against them, and prosecuted or put them under surveillance upon their return.

Then came Iraq's 1990 invasion of Kuwait, and in response the Saudi "betrayal," when the kingdom allowed US attacks to be launched from its bases.

The gulf war proved another seminal point in Mr bin Laden's road to international pariah status. Unwelcome in his own country, he went to Sudan, whose intellectual leader, Hassan Tourabi, had been calling for the total defeat of the Iraqis.

Mr Tourabi's brand of extremism, which received wide publicity at the time, was premised on the theory that only total defeat of Iraq and the complete discrediting of "secular" Arab regimes which he regarded as traitors to Islam, could lead to a progression of events that might bring about the collapse of un-Islamic Arab establishments and create the opportunity to set up a "pure" Islamic government.

Mr Tourabi, according to Mr Salhi, was more a politician and opportunist than intellectual. He used Mr bin Laden, who became a member of the Tourabi family when he married one of his nieces, to finance the construction of roads and an airport, but he was not an intellectual influence on Mr bin Laden.

Nevertheless, Mr bin Laden took Mr Tourabi's message, and set about training terrorists in urban warfare.

One of Mr bin Laden's earliest actions was arguably also one of his most successful. US prosecutors, claim that al-Qaeda trained those who ambushed and killed 18 US servicemen stationed in Somalia in 1993.

Mr bin Laden's goal to force the US out of Africa worked. Since then US soldiers have not returned to Africa despite numerous United Nations peacekeeping missions to the troubled continent. His mission to drive US troops out of the Middle East has not been as successful.

Conversely, it is Mr bin Laden who has been driven into exile. In 1994 Saudi Arabia stripped him of his nationality after Egypt, Yemen and Algeria had all accused him of bankrolling militancy in their countries. Sudan provided him with a passport, and allowed him to stay for another two years until, under US pressure, the Sudanese government asked him to leave.

He returned to the sanctuary of the Taliban, whose sovereignty is recognised by only Pakistan, Saudi Arabia and the United Arab Emirates.

Mr bin Laden is said to be behind more than a dozen of the most serious terrorist incidents since 1993 in Somalia, New York, Saudi Arabia, Egypt, East Africa and Yemen.

Still, pinpointing Mr bin Laden's exact involvement - knowing whether he merely inspired an attack, trained militants, or planned and ordered it - remains a difficult task.

Mr bin Laden has at least six sources of funding, analysts say. Firstly, his own capital. Secondly, US and Saudi funds which were diverted or retained from the last days of the campaign against the Soviets. Then there are the contributions from other Islamist groups in Islamic states gathered from their own domestic constituencies. Similar funds from Western-based Islamic groups, notably in the US and Britain, where they flourish legally. Money diverted from charities which are the recipients of Gulf Arab states' donations and then there is the money from wealthy Gulf Arab individuals.

Neither publicity nor financial profit, one analyst said, "is important to a man seeking a martyr's death. His conviction and the act itself assure him a place in paradise.

The family he leaves behind is also rewarded, with enhanced honour and prestige in its own community and the promise of eternity in paradise. These are more important than money."