The death of Osama bin Laden means someone new will vault to the top of the FBI's Most Wanted Terrorist list as well as the Al-Qaeda food chain. By all accounts, that man seems to be Ayman al-Zawahiri.

Zawahiri was the al-Qaeda network's unofficial number two man and early reports indicate he could be promoted to number one with bin Laden's death. Zawahiri is already on the FBI's Most Wanted Terrorists list for his direct involvement in the bombings of the United States Embassies in Dar es Salaam, Tanzania, and Nairobi, Kenya. The United States Department of State is offering a reward of up to $25 million for information leading to his arrest or apprehension.

Much like bin Laden, the Cairo-born Zawahiri has been in hiding since the September 11, 2001 terrorist attacks. In a biography on bin Laden, Pakistani author and journalist Hamid Mir said al-Zawahiri was the real brains and mastermind behind the Al-Qaeda network as well as the September 11 attacks.

Zawahiri has released several videos and messages over the years announcing threats and warnings to the Western powers; most specifically the United States. His last known message was in 2009, when he urged his supporters to seek a jihad against the Jews, as well as America and the West. In July of 2009, he released a message that urged Pakistani people to support the Taliban. In June of that year, he released a tape saying President Barack Obama was not welcome in Egypt.

According to an article in The Wall Street Journal, a senior official in the Obama administration said Zawahiri is likely to take over, but will have trouble maintaining the same influence that bin Laden had.

Although al-Qaeda may not fragment immediately, the loss of bin Laden puts the group on a path to decline that will be difficult to reverse, the official said according to The Wall Street Journal.

Zawahiri was interested in radicalism from an early age. He joined the Muslim Brotherhood, a fundamentalist group, when he was 15. In the early to mid-1980s, he was arrested along with hundreds of others following assassination of former Egyptian President Anwar Sadat. He left Egypt in 1985 for Saudi Arabia.

After that, he went to Peshawar, Pakistan, where he met bin Laden when the latter was running a base for mujahedeen, the Muslim guerrilla warriors who cut their teeth fighting the Soviet Union in Afghanistan. In 1998, he issued a joint fatwa with bin Laden called the World Islamic Front Against Jews and Crusaders.