The man considered the leader of Al Qaeda (search) in Saudi Arabia, who was killed in a clash with security forces in Riyadh on Friday, took up the Muslim extremist war when he was a teenager and battled around the world.

Abdulaziz Issa Abdul-Mohsin al-Moqrin (search), 31, was believed to be the leader of the group calling itself Al Qaeda in the Arabian Peninsula that claimed responsibility for beheading American hostage Paul M. Johnson Jr. Johnson's death was reported Friday.

Al-Moqrin's fighters are blamed for a recent string of terror strikes in Saudi Arabia, among them the May 29 shooting and hostage-taking attack on the oil hub of Khobar that killed 22 people, most of them foreigners, and the Nov. 8, 2003, suicide bombing at Riyadh housing compounds that killed 17, mostly non-Saudi Muslims working in the kingdom.

Al-Moqrin was born in Shagra, a village 100 miles northeast of the capital, Riyadh. Saudi newspapers have reported he later lived in Sweidi, a district in Riyadh known as a hotbed of Islamic extremism.

A BBC cameraman was shot and a correspondent wounded while reporting in Sweidi earlier this month.

Evan Kohlman, an expert who maintains an Internet database on terrorism, said al-Moqrin was a "dropout from the Saudi educational system" who fought with other Saudis against the Soviets in Afghanistan in the late 1980s.

Saudi security officials say al-Moqrin trained with Usama bin Laden (search) in Afghanistan and later fought in Bosnia and Algeria.

Saudi security officials say al-Moqrin was arrested in 1999 for illegally crossing the border into Yemen and was sent home, where he spent two years in prison.

Saudi security officials say al-Moqrin took over Al Qaeda operations in the kingdom after his predecessor was killed by security agents earlier this year, but he had masterminded attacks before that.

Al-Moqrin's predecessor, Khaled Ali Haj (search), a Yemeni, had succeeded Youssef al-Airi, who was killed in a clash with Saudi security forces in early 2003.

Al-Moqrin, known as a smart and brutal tactician, was the most-wanted militant in Saudi Arabia. His attacks in recent months have shown tactical flexibility — devastating car bombs as well as pinpointed strikes like the kidnapping of Johnson, a first.

The militants also have committed acts apparently meant to sow fear — such as tying the bodies of their victims to cars and dragging them through the streets — and have used the Internet to spread that fear.

An Internet statement last month purportedly from al-Moqrin said Al Qaeda relies on independent cells that function without "organizational cohesion." The statement said Al Qaeda cells follow the group's example as well as books and periodicals on how to carry out attacks.

In another Internet statement attributed to Al-Moqrin, he indicated he was recruiting Saudis to fight the U.S.-led occupation army in neighboring Iraq.

"By sending our fighters to Iraq, we are not only serving the Iraqi cause, but the cause of Islam," the statement said.

A video that surfaced on Web sites in April showed a masked man identified as al-Moqrin vowing to avenge the slaying of Muslims in Iraq and Afghanistan by expelling Americans from the Arabian Peninsula.