While bin Laden was reclusive, al-Zawahiri was the public face of al-Qaeda, and repeatedly denounced the United States and its allies in video messages. Like bin Laden, al-Zawahiri has long been thought to be hiding along the Afghan-Pakistan border. Al-Zawahiri was born in 1951 into an upper-class family of scholars and doctors. An eye surgeon, al-Zawahiri joined the militant Egyptian Islamic Jihad group when it was founded in 1973 and was among 301 people arrested following the assassination of Egyptian President Anwar Sadat in 1981. He went on trial but was cleared of involvement in the assassination.

He met bin Laden in the mid-1980s in Peshawar, Pakistan. They were both there to support mujahideen guerillas fighting the Soviets in Afghanistan. Taking over the leadership of Islamic Jihad in Egypt in 1993, he was a key figure in a campaign in the mid-1990s to set up a purist Islamic state there. More than 1200 Egyptians died. Al-Zawahiri has been described as al-Qaeda's chief organiser and bin Laden's closest mentor. The world had already moved beyond bin Laden and al-Qaeda. Operationally al-Qaeda's command and control had been crippled and their top leaders had either been arrested or killed "Al-Zawahiri was always bin Laden's mentor, bin Laden always looked up to him," terrorism expert Bruce Hoffman of Georgetown University told the Associated Press.

"He spent time in an Egyptian prison, he was tortured. He was a jihadi from the time he was a teenager, he has been fighting his whole life and that has shaped his world view," Professor Hoffman added. In recent times, al-Qaeda in the Arabian Peninsula (AQAP) has been seen as posing the biggest threat to the United States. AQAP has taken credit for the 2009 Christmas Day "underpants bomb" plot, in which a passenger tried to blow up a plane travelling from Amsterdam to Detroit in the United States, and for last year's attempt to place bombs in printer cartridges on FedEx and UPS cargo planes. AQAP is already operating independently of bin Laden's organisation, Yemen expert at Princeton University Gregory Johnsen told the Christian Science Monitor. "Nasir al-Wuhayshi is running AQAP as a parallel organisation, in that Wuhayshi has command and control of people who swear an oath of allegiance to him, in much the same way that they used to for Osama bin Laden."

Al-Wuhayshi, from the southern Yemeni governorate of al-Baidaa, is a former secretary of bin Laden who fought with him in Tora Bora, Afghanistan, in 2001. He fled to Iran from Afghanistan before being extradited and returned to Yemen in 2003. Al-Wuhayshi escaped from a maximum-security prison in 2006 and oversaw the 2009 merger of the Saudi and Yemeni branches of al-Qaeda. Qasim al-Raymi, AQAP's military commander, also escaped with al-Wuhayshi (and 20 other prisoners) and orchestrated the creation of the new organisation. Anwar al-Awlaki, a regional commander in AQAP, has also been cited by US officials as an influential terrorist.

An American citizen, al-Awlaki was in email contact with the US army psychiatrist accused of killing 12 soldiers and one civilian at Fort Hood, Texas, in 2009. Last year, the chairwoman of the US House Homeland Security Subcommittee on Intelligence, Jane Harman, described the English-speaking, internet-savvy Muslim cleric as "probably the person, the terrorist, who would be terrorist No.1 in terms of threat against us". US President Barack Obama authorised the CIA to kill al-Awlaki in April last year, and the 40-year-old survived an air strike in southern Yemen. Al-Awlaki was also involved in talks with an Australian group, the Sydney Muslim Youth, to deliver a sermon to young believers. Another American, 32-year-old Adam Yahiye Gadahn, is No.2 on the FBI's most-wanted list after bin Laden.

Gadahn, who was born in Oregon under the name Adam Pearlman, lived in California and converted to Islam when he was 17. He has been called al-Qaeda's American spokesman and is accused by US officials of "treason and material support to al-Qaeda". Another al-Qaeda member growing in prominence is Khalid al-Habib, who has risen reportedly within the ranks of the terrorist organisation from field commander in south-east Afghanistan to become its military commander in 2008. Al-Habib is believed to be either Egyptian or Moroccan. A successor could also come from within the bin Laden family.

Question marks hang over whether one of bin Laden's sons, Saad bin Laden, who has been described as active in al-Qaeda, is still alive. He allegedly facilitated communication between al-Zawahiri and an elite unit of Iran's Revolutionary Guards. In July 2009, US media reported Saad might have been killed in a missile strike in the Federally Administered Tribal Areas (FATA) of north-west Pakistan. Al-Qaeda: becoming irrelevant? Counterterrorism experts have also pointed out that al–Qaeda as a terrorist organisation has increasingly lost its relevance, especially in light of the recent uprisings in the Middle East and North Africa. But that is not to say that fanatics don't still believe in terrorism.

"The world had already moved beyond bin Laden and al-Qaeda. Operationally al-Qaeda's command and control had been crippled and their top leaders had either been arrested or killed," London School of Economics's al-Qaeda expert Fawaz Gerges told Reuters. "More importantly, al-Qaeda has lost the struggle for hearts and minds in the Arab world and elsewhere and has had trouble attracting followers and skilled recruits." Olivier Roy, an expert on Islamist militancy at the European University Institute in Italy, told Time magazine that Arab youths had moved on from al-Qaeda's rhetoric. Loading "It's certainly coincidence that the two events are linked in time, but in fact it's logical because the death of bin Laden symbolised the marginalisation of al-Qaeda in the Middle East," Professor Roy said.

- with agencies