'Ayman is to Osama as the brain is to body'

As mentor to Osama bin Laden and the brains behind al Qaeda, Ayman al Zawahri is seen as one of the world's most dangerous terrorists.

Many believe he was the operational brains behind the September 11 terrorist attacks on the US in 2001.

He is also thought to have been behind the 1998 truck bombings at US embassies in Kenya and Tanzania, which killed 224 people and injured thousands.

If he is killed or captured now it will be a bitter blow for the terror network and its leader.

He is considered Bin Laden's right-hand man, but many experts believe he is also the terror chief's role model and inspiration.

The combination of wealthy Saudi Bin Laden's money and al Zawahri's ideology led to the creation of al Qaeda. "Ayman is for Osama like the brain to the body," said Cairo lawyer Montasser al Zayat, who has represented many Islamic militants.

Born in Egypt in 1951, al Zawahri comes from a middle-class family. His father was a pharmacology professor, his grandfather the grand imam of Cairo's Al Azhar university.

He graduated from Cairo University's medical school in 1974 and has a masters degree in surgery.

His radical thinking had already blossomed. At 15 he was arrested for being a member of the outlawed Muslim Brotherhood, the oldest fundamentalist group in the Arab world.

He served three years in jail for his part in the 1981 assassination by Islamic militants of Egyptian president Anwar Sadat. He later left for Saudi Arabia, and met Bin Laden in 1985 during the Afghan war against Russian occupation. Mr al Zayat told Time magazine: "When Osama went to Afghanistan, he was just a young man supporting the Afghans.

"He did not have a political outlook. Ayman controlled him completely. He convinced him of the principles of jihad."

Al Zawahri has also controlled finances and operations for al Qaeda.

"Ayman is much more politically skilled than Bin Laden," Mary Anne Weaver, author of A Portrait of Egypt: A Journey Through the World Of Militant Islam, told Time.

"He's better educated. He has a larger world view."

Well travelled, he is thought to have lived in Denmark and Switzerland in the early Nineties.