Egyptian-born doctor and surgeon Ayman al-Zawahri is al Qaeda's second-in-command expected to succeed Osama bin Laden following his killing in a firefight with U.S.forces in Pakistan.
Zawahri has been the brains behind bin Laden and his al Qaeda network, and at times its most public face, repeatedly denouncing the United States and its allies in video messages.
In the latest monitored by the SITE Intelligence Group last month, he urged Muslims to fight NATO and American forces in Libya.
"I want to direct the attention of our Muslim brothers in Libya, Tunisia, Algeria, and the rest of the Muslim countries, that if the Americans and the NATO forces enter Libya then their neighbours in Egypt and Tunisia and Algeria and the rest of the Muslim countries should rise up and fight both the mercenaries of Gaddafi and the rest of NATO," Zawahri said.
Born into an upper-class family of scholars and doctors in an upscale Cairo neighbourhood, the cerebral Egyptian in his late-50s is second after bin Laden on the FBI "most wanted terrorists" list.
Both bin laden and Zawahiri eluded capture when U.S.-led forces toppled Afghanistan's Taliban government in late 2001 after al Qaeda's Sept. 11 attacks on U.S. cities.
But on Sunday bin Laden was killed in a firefight with U.S. forces and his body was recovered, U.S. President Barack Obama said. There was no word on Zawahri.
Bespectacled, with grey hair and a grey beard, Zawahri won prominence in Nov. 2008, when he attacked then U.S. President-elect Obama as a "house Negro," a racially-charged term used by 1960s black American Muslim leader Malcolm X to describe black slaves loyal to white masters.
In a subsequent video, in Sept. 2009, Zawahri returned to the attack on Obama, saying he was no different from his predecessor George W. Bush.
"America has come with a new deceptive face ... It plants the same dagger as Bush and his predecessors did. Obama has resorted to the policies of his predecessors in lying and selling illusions," said Zawahri, clad in white robe and turban.
Like bin Laden, Zawahri has long been thought to be hiding along the rugged Afghan-Pakistan border. The last video of Zawahri and bin Laden together was broadcast by al Jazeera on Sept. 10, 2003. It showed them walking in mountains, calling for jihad and praising the Sept. 11 hijackers.
"BRAIN TO THE BODY"
Analysts have described Zawahri as al Qaeda's chief organiser and bin Laden's closest mentor. "Ayman is for bin Laden like the brain to the body," said Montasser al-Zayat, a lawyer in Cairo who once represented Zawahri.
In a video after the Sept. 11 attacks, Zawahri called them a "great victory" achieved "thanks to God".
He has not always been so ebullient.
As U.S.-led forces drove out the Taliban in 2001, Afghan sources described him flying into a fury at the nonchalance of Taliban fighters playing badminton behind the front lines while U.S. bombs rained from the skies.
Zawahri and bin Laden met in the mid-1980s when both were in the Pakistani city of Peshawar to support guerrillas fighting the Soviets in Afghanistan, and worked closely thereafter. But the alliance was not Zawahri's first foray into militancy.
Born in 1951, he was the son of a pharmacology professor and grandson of the grand imam of Al Azhar, one of the most important mosques in the Muslim world.
He graduated from Egypt's most prestigious medical school in 1974 and did a second degree in surgery. By then he was involved with the Muslim Brotherhood, a non-violent group seeking the creation of a single Islamic state.
When the militant Egyptian Islamic Jihad was founded in 1973, he joined. When members posed as soldiers and assassinated President Anwar Sadat in 1981, he was among 301 people arrested.
He went on trial but was cleared. He did, though, spend three years in jail for having an unlicensed pistol. On his release, Zawahri made his way to Pakistan where he worked with the Red Crescent treating fighters wounded in the Afghan war.
Taking over the leadership of 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, in which more than 1,200 Egyptians died.
In 1999, an Egyptian military court sentenced Zawahri to death in absentia. He has also been indicted in connection with the 1998 bombings of U.S. embassies in Kenya and Tanzania.
Days after those bombings, he telephoned a Pakistani reporter, denying responsibility, but urging Muslims to "continue their jihad against the Americans and Jews".
An hour later, U.S. cruise missiles hit al Qaeda's Afghan training camps. Both bin Laden and Zawahri escaped injury.
Zawahri's wife, Azza, and three daughters were reported killed in a bombing strike on the Afghan city of Kandahar, the stronghold of the Taliban, in early December 2001.
Zawahri has appeared regularly in a series of video or audio messages since then, criticising the U.S. war in Iraq, praising the Taliban and the suicide bombers who attack London in 2005 and urging Muslims to help victims of an earthquake in Pakistan.
He has occasionally shown he is aware of criticisms by Muslims who dislike the group's indiscriminate violence.
"SEIZE A STATE"
In a 2005 letter to Iraq al Qaeda leader Abu Musab al-Zarqawi, Zawahri suggested it was time to end beheadings of captives and start acting as more of a political leader in anticipation of the eventual U.S. withdrawal from Iraq.
In 2008, Zawahri held an unprecedented question and answer session online with al Qaeda sympathisers who repeatedly questioned him over the group's killings of civilians in Iraq.
Zawahri in his responses denied killing innocents, and said that if any died in attacks it was through error or necessity, for example if they were being used as human shields Zawahri has repeatedly called for al Qaeda to seize control of a state, a goal the group has never come close to despite its alliance with Afghanistan's late 1990s Taliban rulers.
"Confronting the enemies of Islam and waging a jihad against them require a Muslim authority, established on Muslim territory that raises the banner of jihad and rallies Muslims around it," he wrote in a 2001 essay, Knights Under The Prophet's Banner.
"If we do not acheive this goal, our actions will be nothing more than small scale harassment."