AMMAN, Jordan -- In a video image posted on the Internet the week before last, a quivering, blindfolded American kneels on the floor of an empty room as five hooded men stand behind him, dressed in black. After reading a speech from a sheaf of white papers, the leader of the group pulls a long knife from his shirt and slices off the captive's head in a well-practiced manner.

The killer is wearing a mask, but he is identified in a statement accompanying the video as Abu Musab Zarqawi. He is the most wanted man in Iraq and at the vanguard of a new generation of Islamic radicals that has confronted the United States and its allies since the invasion of Iraq 18 months ago.

Although Zarqawi has assembled temporary alliances with Osama bin Laden's al-Qaida network over the years, evidence shows that he has always sought to forge his own path with a largely distinct, if occasionally overlapping, agenda.

Zarqawi and his group, Monotheism and Jihad, have become best known for helping fuel the insurgency in Iraq. But according to European and Arab intelligence officials and counterterrorism specialists, he has never abandoned his primary goals: to topple the monarchy in his native Jordan and to attack Jewish targets in Israel and around the world. As Zarqawi has become more prominent in recent years, he has expanded his original sphere of influence in the Middle East by forming cells in Europe.

Skeptics say that the U.S. government has transformed Zarqawi into a larger-than-life figure by exaggerating his capabilities and using him to personify the Iraqi resistance, which has many factions and appears to rely mainly on Iraqi fighters, not on foreigners. But Zarqawi also has helped to enhance his own legend by embracing tactics that have generated enormous publicity.

In May, he personally inaugurated a wave of hostage-takings and beheadings in Iraq by decapitating Nicholas Berg, 26, a businessman from Pennsylvania, and posting the videotaped episode on the Internet. Last week, the kidnapping trend reached a new zenith when he and his followers posted videos on the Internet showing the decapitations of two other Americans, Eugene "Jack" Armstrong, 52, a native of Hillsdale, Mich., and Jack Hensley, 48, of Marietta, Ga. The group, which had also taken a Briton hostage, warned that he could meet the same fate.

Almost every week, U.S. forces in Iraq bomb or blow up suspected Zarqawi hide-outs and safe houses, but so far they have been unable to corral the 38-year-old Jordanian. The United States has placed a $25 million bounty on his head, the same reward offered for bin Laden.

Zarqawi was barely known outside Jordan until a year and a half ago, when Secretary of State Colin Powell identified him as a bin Laden "collaborator and associate." In a speech to the United Nations, Powell cited Zarqawi's presence in Baghdad as evidence that Iraq's president, Saddam Hussein, had struck an alliance with al-Qaida, a claim that became a major part of the Bush administration's argument for going to war.

"He was and is the leading figure of al-Qaida in Iraq," said a Jordanian security official, who agreed to an interview on the condition of anonymity. "He is now the head of the pyramid of terrorism in Iraq, and he does have the ability and psychology to replace bin Laden."

Zarqawi also has been accused by some European and Arab authorities of orchestrating plots to cause mass casualties throughout Europe and the Middle East, including in Germany, France, Britain, Spain, Morocco, Tunisia, Jordan and Turkey. But some European intelligence officials say that claims about Zarqawi's reach are overblown. They contend that extensive investigations have turned up no evidence that he had a hand in some of the attacks attributed to him, such as the bombings in Madrid, Spain, and Istanbul, Turkey, in the past year.

Zarqawi is a member of the Bani Hassan tribe, according to intelligence officials. His real name is Ahmed Fadhil Nazzar Khalaylah, an identity that he abandoned several years ago when he renamed himself after his hometown, Zarqa, an industrial city 17 miles northeast of Amman.

He grew up in a family of modest means and was a troublemaker from an early age, dropping out of high school and repeatedly getting into drunken brawls, intelligence officials say. In the late 1980s, he went to Afghanistan to join Islamic radicals who had been fighting Soviet troops there.

Upon his return home a few years later, he helped start a local Islamic militant group called Jund al-Sham, which quickly attracted the attention of Jordanian authorities. In 1992, he was sent to prison, where he adopted more radical Islamic beliefs, according to Jordanian officials and acquaintances.

Seven years later, he was released in a general amnesty by Jordan's King Abdullah. Within months, according to Jordanian officials, Zarqawi tried to resurrect his Jund al-Sham organization and became involved in what became known as the millennium plot, a bid to bomb the Radisson SAS Hotel in Amman and several tourist sites in Jordan just before New Year's Day 2000. But the plot was discovered in its late stages, and Zarqawi fled to Pakistan. That year, with his visa revoked by Pakistani authorities, he crossed the border into Afghanistan and made his first contacts with al-Qaida leadership.

According to Jordanian officials and court testimony by jailed followers in Germany, Zarqawi met in Kandahar and Kabul with bin Laden and other al-Qaida leaders. He asked for assistance and money to set up his own training camp in Herat, near the Iranian border.

With al-Qaida's support, the camp opened and soon served as a magnet for Jordanian militants. In mid-2001, he returned to Kandahar to ask al-Qaida to finance a plan for his fighters to infiltrate Israel, according to a U.S. Treasury Department report. In early September, a few days before the hijackings in the United States, he met in Iran with a Jordanian ally and ordered him to set up a cell in Germany to strike Jewish targets there, according to files compiled by German investigators. German police broke up the group before it could carry out any attacks.

About a month later, Zarqawi was back in Afghanistan and joined Taliban and al-Qaida fighters resisting the U.S.-led invasion. In late 2001, he was wounded in the chest during a firefight, according to a Jordanian intelligence source.

Zarqawi's whereabouts in 2002 are difficult to pin down, although Western and Arab intelligence agencies say he moved frequently among Iran, Syria, Lebanon and Iraq, expanding his network.

Powell, in his U.N. speech, said Zarqawi arrived in Baghdad in March 2002 for medical treatment and stayed two months. During his convalescence, Zarqawi was joined by a dozen followers who moved money, supplies and al-Qaida-affiliated fighters throughout Iraq, Powell added.

About the same time, Jordanian authorities indicted Zarqawi in absentia for his role in the millennium plot and issued a warrant for his arrest. Jordanian investigators had followed his trail to Iraq and tried to persuade Saddam's government to extradite him.

In the summer of 2002, according to Jordanian court documents, Zarqawi organized a plot to attack Western and Jewish targets in Jordan and began training a small band of fighters at a base in Syria. On Oct. 28, 2002, the group staged its first strike, fatally shooting U.S. diplomat Laurence Foley, a senior administrator of the U.S. Agency for International Development, in Amman.

The Jordanian indictment in that case alleges that Foley's assassins met with Zarqawi in Syria and received money for the operation from his network in Iraq. Despite evidence of his presence in their country, the Syrians ignored requests from the United States and Jordan to extradite Zarqawi, according to Arab intelligence sources.

Not long after, Zarqawi found refuge again in a third country in the region, Iran. In February 2003, Zarqawi met at a safe house in eastern Iran with Mohammed Ibrahim Makawi, al-Qaida's military chief, an Egyptian who is known as Saif Adel, and they discussed strategies for combining forces in Iraq to resist the anticipated U.S. invasion, Arab intelligence sources said. By this time, Zarqawi's attention was focused squarely on Iraq. In March, British intelligence warned that his network had set up sleeper cells in Baghdad to mount a resistance to the U.S. occupation, according to a report made public this summer in London.