In the aftermath of bin Laden's killing, the United States and its allies in the struggle against Islamist extremist groups look to strike a further blow against al-Qaeda, such as catching or killing his deputy.
Zawahiri, who met bin Laden in the 1980s when both men joined the fight against the Soviet Union's occupation of Afghanistan, may be even more difficult to find, since he is known for blending in with the masses in Pakistan. The U.S. government is offering a $25 million reward for information.
It was Zawahiri who urged bin Laden to deploy suicide attackers against the West, persuading him that using the Palestinian tactic - forbidden by the Saudi brand of Islam on which bin Landen was raised - is permitted "martyrdom," said Fawaz Gerges, author of the 2005 book The Far Enemy: Why Jihad Went Global. He has been seen by terrorism specialists as bringing organizing skills and strategic thinking to the core al-Qaeda group.
Zawahiri's main contribution to the movement in recent years has been a series of video and audio messages, said Dan Byman, a counterterrorism expert at the Brookings Institution.
While the messages are "small, rambling tidbits," Byman said, "they are important because they show the organization is still active, and they show a strategic plan of what to target and they enthuse a new generation of recruits."
Locating Zawahiri, 59, may be tough. Unlike bin Laden, whose height and thin build set him apart in crowds, Zawahiri has frustrated U.S. forces with his ability to hide in Pakistan's cities. The FBI lacks information about his height, weight, or build.
Zawahiri was wanted in the United States even before the Sept. 11 attacks. He was indicted in absentia in 1999 for the 1998 bombings of U.S. embassies in Tanzania and Kenya that killed 224 people. He was also considered the mastermind of the October 2000 bombing of the USS Cole in Yemen, in which 17 sailors died.
U.S. officials believe Zawahiri is in Pakistan, said Byman, a former staff member of the 9/11 commission.
"Zawahiri will be very careful of his personal security, but . . . he needs to be out there," Byman said. "In his new capacity, he needs to meet and communicate, and then he's vulnerable."