''The truth is, we simply don't know what would happen if bin Laden is killed or captured. But I'm willing to bet that whatever happens, it would work in our favour.''

This was the view of then CIA chief Michael Hayden in November 2008. The trail to ''UBL'' - the Arabic-English transliteration by which Osama bin Laden was known in officialdom - was long cold. But in the hope of eventually tracking down the elusive al-Qaeda leader, intelligence agencies were still thinking about what might happen next.

''This is an organisation that has never been through a change at the top,'' Hayden said. ''For 20 years, bin Laden has been the visionary, the inspiration or harmonising force behind al-Qaeda.''

And now, finally, he is gone.

What bin Laden's death means for the future of al-Qaeda is now the question coursing through the halls of spy agencies around the world. And perhaps as importantly, who, if anyone, will now seek to take his place?

The terrorist organisation is already a vastly different outfit from a decade ago, when it ran training camps in the tribal lands of Afghanistan and found political sanctuary with the Taliban. The al-Qaeda leadership - widely known by the designation ''AQ core'' - was scattered by the invasion of Afghanistan in October 2001. The network spread to a number of affiliates in regions across the globe; there is al-Qaeda in the Arab Peninsula, al-Qaeda in Iraq, al-Qaeda in the Islamic Maghreb, and al-Shabab based in Somalia, sometimes known as al-Qaeda in East Africa.

Al-Qaeda began as a cobbled together collection of different factions, but bin Laden became the focus of their allegiance. He was the titular head, offering broad guidance to his followers in irregular statements released on the internet. He had the deadly credentials as a leader. Even in death, the zealots will revere him as a martyr.

Most terrorism analysts believe the mantle of al-Qaeda leader will pass to bin Laden's deputy, Ayman al-Zawahiri. He was always HVT2 - or ''high value target two'' - after bin Laden for American military and security agencies. Now he is No. 1. An Egyptian medical doctor, Zawahiri founded a group known as Egyptian Islamic Jihad and in the mid-1990s was accused of attempting to assassinate Egypt's former president Hosni Mubarak. Before that he had spent time in jail suspected of a plot against Mubarak's predecessor, Anwar Sadat. Zawahiri had a profound ideological influence on bin Laden, taking the rich young scion of a Saudi building magnate, who went to Afghanistan in the 1980s to fight the Russians, and turning him towards the fight against Arab rulers and eventually, the West.

''He had a lot of anger. Bin Laden and Ayman were very close friends,'' wrote Egyptian journalist Faraj Ismail, who met Zawahiri in the 1980s and is quoted in the book by Peter Bergen, The Osama bin Laden I Know. ''Ayman was more mature politically and religiously than Osama. Ayman was the one who got Osama to focus not only on the Afghan jihad, but regime change in the Arab world.''

Despite the $US25 million ($A23.4 million) bounty on his head - equal to the reward offered for bin Laden - Zawahiri has been a far more vocal presence over past decade. He speaks good English, though rarely, and and has released hours of lengthy sermons to news services or the internet.

Bin Laden, by contrast, could go months, even years, without being heard.

Zawahiri's profile put him at risk. A suspected US missile strike inside the Pakistani tribal province of Waziristan in early 2006 is thought to have missed him by moments - he may have been injured in the attack. He stopped appearing in open terrain during his videos, lest intelligence agencies pick up on his location. But Zawahiri has remained one of the most vocal of al-Qaeda's leaders, shown earlier this year as he presented his views on politics in Egypt.

Democracy, he said, ''means that sovereignty is to the desires of the majority, without committing to any quality, value or creed. A democratic state can only be secular, meaning non-religious.'' And that, according to the violent world view of al-Qaeda, is the enemy.

But Zawahiri has doubters. His rhetorical style is that of a rabble-rouser, a firebrand who lacks the lyrical subtleties of bin Laden's pronouncements. Zawahiri is seen as a poor manager, quick to anger and provoke resentment, especially from non-Egyptian members of al-Qaeda, who see him as self-centred and arrogant. A Saudi paper yesterday claimed that bin Laden had been betrayed by his deputy after a ''simmering internal power struggle'' - but rumours of this kind are hard to substantiate.

Zawahiri was born in 1951. His parents were described as aristocratic in a 2002 biography written by Egyptian lawyer Montasser al-Zayyat. ''His family noticed his interest in reading, academic excellence and studiousness from a young age,'' Zayyat wrote. Zawahiri graduated from Cairo University with a medical degree in 1974 with the highest grades, later earning a master's degree in surgery. Then came his imprisonment in 1981, torture and stronger ties to radical groups.

But his didactic style, constant references to Egypt as the centre of the Islamic nation, and favouritism of Egyptian militants reportedly frustrated other al-Qaeda members. This has led some analysts to wonder whether Zawahiri can achieve the same authority as bin Laden.

''Although al-Qaeda will not fragment immediately,'' one US official was quoted as saying, ''the death of bin Laden puts al-Qaeda on a path of decline that will be difficult to reverse.''

The hope is to see a repeat of the experience in Iraq, when the local al-Qaeda commander Abu Musab al-Zarqawi - who for a time rivalled both bin Laden and Zawahiri for influence among violent Islamists - was killed. The group fractured under pressure, unable to sustain the fight against US forces.

The present CIA chief Leon Panetta said the death of bin Laden gave the US a chance to press the attack and sow confusion in the terrorist ranks. ''I can assure you whoever takes his place, he will be No. 1 on our list.''

What might lead to possible fragmentation of al-Qaeda is the increased prominence of other terrorists challenging the isolated old guard, such as the Yemeni-American Anwar al-Awlaki with the Arabian Peninsula branch. But Awlaki is not a tactical leader directing operations. His role is more propagandist, railing against the West. What is also possible is no single leader will emerge, but rather a diverse range of commanders will compete to outdo each other with atrocities.