Al Qaeda leader Dr. Ayman al-Zawahiri, Osama bin Laden's top lieutenant, may be the most influential member of al Qaeda.
The Egyptian surgeon has been called one of the masterminds behind the vast al Qaeda network and a likely successor to bin Laden. And as former head of Egyptian Islamic Jihad, he has a long history of terror-related crimes.
Recent intelligence reports indicate Zawahiri and bin Laden have moved out of Pakistan, crossing the mountainous border back into Afghanistan.
Those reports have came as Arabic television stations aired audiotapes purportedly made by Zawahiri. In one of the tapes, a man thought to be Zawahiri taunted President Bush and threatened new attacks on the United States.
"Bush," said the voice believed to be Zawahiri's, "fortify your targets, tighten your defense, intensify your security measure because the fighting Islamic community, which sent you New York and Washington battalions, has decided to send you one battalion after the other, carrying death and seeking heaven."
Top-Level Arrest Warrant
Interpol issued a top-level arrest warrant for Zawahiri shortly after the terror attacks of Sept. 11, 2001, yet his whereabouts have long been unknown. The list of terrorist attacks tied to al-Zawahiri is long:
He was indicted in New York two years ago in connection with the bombing of U.S. embassies in Tanzania and Kenya in August 1998. The FBI believes he may be the one who ordered the massive bombings.
He served three years in prison in Egypt on charges connected to the 1981 assassination of Egyptian President Anwar Sadat. Since then, he has been sentenced to death by Egypt in absentia.
He was suspected of helping organize the 1997 massacre of 67 foreign tourists in the Egyptian town of Luxor.
He helped fund and organize the attack on the Egyptian embassy in Islamabad in 1995.
'More Dangerous of the Two'
Zawahiri is regarded by many investigators as smarter and more dangerous than bin Laden, himself.
"To America and the West, Ayman Zawahiri is definitely the more dangerous of the two," said Mohammad Salah, a reporter for Al Hayat, in Arabic. "To Islamic fundamentalists, he is much more important ideologically."
After U.S. airstrikes against Afghanistan in 1998, it was Zawahiri who told the Muslim world: "The war has begun. Americans should wait for an answer."
The attacks on the World Trade Center and the Pentagon in 2001 may have been that answer.
It's a long distance to the rugged wilderness of Afghanistan from the quiet green suburb in Cairo where Zawahiri grew up in a family that was peaceful and pious, prominent and prosperous. His drift to a fraternity of violence is rooted in one event — Camp David.
For Zawahiri, the 1977 treaty between Egypt and Israel meant Anwar Sadat was a traitor. Sadat was assassinated over 20 years ago, and Zawahiri was among scores of Muslim fundamentalists implicated in the murder.
At their trial, he was defiant. "We are here, the real Islamic front. We are here, the real Islamic opposition against Zionism, communism and imperialism," he ranted in a holding cell in court.
Connection to Bin Laden
Zawahiri grew up in a well-connected family. One grandfather was a noted Muslim cleric, the other an ambassador. Zawahiri became a surgeon and one of the first upper class Egyptians to join the country's militant movement, Islamic Jihad.
After three years in prison, during which he was tortured, he joined bin Laden in Afghanistan in 1986 as his personal physician and a mentor in terrorism.
"Ayman Zawahiri and Osama bin Laden [have] the same ideology — to defend in their terms their religion and their land," says Dia'a Rashwan, an analyst at the Al Ahram Center for Strategic Studies.
In 1998, Zawahiri was one of five Islamic leaders to sign on to bin Laden's declaration calling for attacks against U.S. citizens. More broadly, he was believed to be the person who most influenced bin Laden years ago to take up a worldwide struggle against perceived enemies of Islam. According to some analysts, Zawahiri helped turn bin Laden from a financial backer of the Afghan resistance into a strong believer in the ideology of jihad.
"For his cause, he would embrace death more than we would embrace life," says Salah in Arabic. "He doesn't care if he dies."
The United States Department of State is offering a $25 million reward for Zawahiri, who was once trained to save lives but is wanted for killing thousands instead.
ABCNEWS' Jim Wooten, Brian Ross, David Scott and Michael Baltierra contributed to this report.