The man wears a black hood on the videotape as he grips an AK-47 and threatens to execute an American hostage. In spite of the disguise, a subtitle gives a name for the speaker, Abdulaziz Muqrin, who Saudi authorities say is the chief organizer of the recent attacks on Westerners in Saudi Arabia.

A Saudi native believed to be in his early thirties, Muqrin is a graduate of jihadi training camps in Afghanistan and a veteran of conflicts in Somalia and Bosnia, as well as Algeria, where he was part of a group known for dismembering the bodies of its enemies on videotape, according to Saudi officials and terrorism analysts.

Even so, he was a relatively unknown Islamic radical until last November, when a small cell loyal to him carried out a suicide bombing at a residential compound in Riyadh that housed mostly Westerners, killing 17 people and injuring 122. The attack landed Muqrin on a most-wanted list of 26 terrorist leaders named by the Saudi government.

Since then, Muqrin's name recognition has soared in the Middle East, partly because many of his rivals from other Islamic underground groups have been killed or arrested and partly because he has displayed an unusual knack for using the Internet to gain public attention even while he remains in hiding.

Muqrin's group, known as al Qaeda in the Arabian Peninsula, most recently has asserted responsibility for kidnapping Paul M. Johnson Jr., 49, an employee of Bethesda-based Lockheed Martin Corp. Investigators say they also believe the group plotted a series of assassinations and other attacks that have killed more than 40 people since November, most of them Westerners.

In a video released on the Internet this week, a hooded man identified on the screen as Muqrin read a statement saying he would execute Johnson in 72 hours, a period that apparently ends Friday, unless Saudi officials released an unspecified number of radicals who have been captured in recent months.

Johnson was kidnapped last weekend in Riyadh at about the same time another American military contractor, Kenneth Scroggs, was killed by gunmen who had followed him home from work on Saturday. A third American, Robert Jacobs, was killed four days earlier in Riyadh under similar circumstances.

Muqrin has also asserted that he organized an attack on a residential and office compound last month in the Persian Gulf city of Khobar. In that incident, four gunmen broke into the complex in search of Westerners, killed 22 people and held dozens of others hostage. Despite being surrounded by Saudi security forces, three of the four assailants escaped, enabling Muqrin to crow in audio clips and commentaries posted on the Internet that his group had taken on the Saudi government and won.

Khaled Batarfi, managing editor of al-Madina newspaper in Jiddah, said Muqrin has quickly become the public face of al Qaeda in the region, even though it is unclear if he has any direct ties to the group headed by Osama bin Laden.

"He's making a name for himself that is now competing with bin Laden," Batarfi said. "He's speaking in his name and saying he has the same principles."

According to Saudi officials and Arab media reports, Muqrin dropped out of school at age 17 and left Riyadh to travel to Afghanistan. He spent about four years there, mostly at a jihadi training camp near Khost. He then took his skills to Africa and Europe, working primarily as a weapons smuggler.

He was arrested in Ethiopia in 1998 for belonging to an Islamic rebel group and extradited to Saudi Arabia, where he was sentenced to four years in prison. His conduct in prison was considered so exemplary -- he spent most of his time memorizing the Koran -- that he was released two years early.

In early 2001, Muqrin returned to Afghanistan. He joined the Taliban in fighting U.S. forces there, then returned to Saudi Arabia a year later. It was then, Saudi security officials said, that he began to establish his own local network of radicals.

Saudi officials say they are not sure whether Muqrin is still in the country. Members of his cell who have been arrested have told interrogators that they do not know his whereabouts and have communicated with him only by e-mail or cellular phone. Some Saudi officials say he remains in Riyadh, where he grew up, while others suspect he is hiding along the Yemeni border.

Prince Turki al-Faisal, the former head of Saudi intelligence and now the country's ambassador to Britain, said security forces have eliminated four of five known al Qaeda cells in the country in the past year. He also expressed confidence that Muqrin's group would be rounded up soon as well.

"Only one al Qaeda cell remains operational in Saudi Arabia," Turki said in an interview this month with Jane's Intelligence Review, a London-based publication. "Even now, it's in the process of being dismantled."

Nawaf Obaid, a security consultant to the Saudi government, said the al Qaeda branch would likely collapse if Muqrin is caught. "He's been able to keep his group mostly intact, until recently," he said. "If you catch him, the whole cell gets destroyed. He's the only leader of his caliber left."

Despite an intensive manhunt, however, Muqrin remains at large. And as his profile has grown, so has his rhetoric. In recent weeks, he has issued grandiose statements about his goals: to chase all Westerners out of the Arabian Peninsula, disrupt world oil markets and topple the Saudi royal family.

Some Saudi officials and terrorism analysts said they doubted that he has the ability to stage major terrorist operations, but warned that he would continue to launch small-scale attacks.

"I would describe it as a case of a wounded tiger," said Mohsen Awajy, a Saudi lawyer and former Islamic radical who now advises the Saudi government on dealing with the underground. "A wounded tiger that knows it's dying but won't go down quietly. He knows he has no choice but to die, so he wants to kill as many people as he can before he does."

Web site video shows a hooded man purported to be Abdulaziz Muqrin threatening to kill an American hostage unless Saudi authorities release jailed radicals.