Abu Musab al-Zarqawi, the Jordanian-born terrorist who was killed yesterday in an American air strike near Baqubah, Iraq, was America’s second most wanted outlaw after Osama bin Laden. He was 39.

Zarqawi was thrust into the international spotlight before the American led invasion of Iraq when Secretary of State Powell denounced him as part of a “sinister nexus” between Iraq and the Al Qaeda guerrilla network and cited his presence in Baghdad as evidence that Saddam Hussein’s regime in Iraq had formed an alliance with Al Qaeda, a claim that became a major part of the coalition case for war.

Well before the attacks of September 11, 2001, Zarqawi concocted a plot to kill Israeli and American tourists in Jordan. He also was accused of being the mastermind behind numerous shadowy terrorist groups on four continents. In Iraq, his mission was not only to destroy the occupation forces, but also to ignite civil war between Sunni and Shiite Iraqis.

He was implicated in a series of atrocities including the beheading of American and British contractors in Iraq. The CIA also accused Zarqawi of involvement in the Madrid railway station bombings of March 2004, and bombings of Shiite worshippers in Iraq the same month.

Despite his notoriety, Zarqawi remained a mystery for most of his career as a terrorist leader identified with the Sunni tradition of Islam. A master of disguise and bogus identities, he was so secretive that even many of his collaborators did not know what he looked like. Some reports alleged that he had lost a leg; others denied the claim. A “wanted” poster issued by the American government listed his height and weight as “unknown.”

Though he was often linked to Al Qaeda, Zarqawi was essentially a “lone wolf” and some experts suggest that claims about his influence were greatly exaggerated. Since most of what is known about him derives from the conflicting accounts of enemies and supporters, it is often difficult to discern the truth from the fiction.

Abu Musab al-Zarqawi (his nom de guerre) was born on October 20, 1966, to an impoverished Palestinian Arab family in a slum neighborhood of Zarqa, a dusty mining town 17 miles north of Amman in Jordan. His real name was Ahmed Fadeel Nazal al-Khalayleh. Zarqawi’s father, Fadel, was a retired soldier and respected elder in the Bani Hassan tribe, one of Jordan’s largest clans, which straddles many borders in the Middle East. Zarqawi’s bloodline was from the poorer side of the Bani Hassan, with no inherited wealth.

As a boy, he was brought up in a cement breeze-block-built house surrounded by mosques. As a teenager, he rebelled against his strict upbringing, and in his hometown he was remembered as a juvenile delinquent who had little to do with religion, despite a strict Islamic upbringing. He became a street fighter, drank alcohol, visited bars, and was once arrested for carrying drugs.

Little is known of Zarqawi’s education, except that he dropped out of secondary school and was barely literate. When he was 20, he went to Afghanistan to join the mujahedeen fighting the Soviet army. There, he was trained in guerrilla warfare, learned about chemical weapons, and came into occasional contact with Osama bin Laden.

Returning to Jordan in 1991 after the Soviet withdrawal, he worked for the local town council as a technician, and joined the Egyptian Islamic Jihad, which merged with Al Qaeda in 1998. He also associated himself with Hizab ut Tahrir, an angry anti-Semitic splinter group devoted to the restoration of an Islamic theocracy.

Arrested and jailed for plotting to overthrow the Jordanian monarchy in 1992, Zarqawi was released under a royal amnesty in 1999. While in prison, he fell under the spell of an extremist cleric, Abu Muhammed al-Maqdisi, the inspiration for a truck bombing of American servicemen in Saudi Arabia in 1996.

Zarqawi’s first known attempted terrorist attack on Western targets was an abortive assault on a European-operated hotel in Amman in 1999. After the attack failed, he fled to Pakistan and settled as a businessman in Peshawar, where his wife and children joined him. Jordan tried Zarqawi in his absence, and sentenced him to death for his part in the alleged plot.

Zarqawi’s family returned home to Jordan after he became estranged from his wife. Later, following Bedouin tradition, he took a second wife, a Jordanian woman he met in Pakistan.

Returning to Afghanistan in 2000, Zarqawi is said to have built his own network of terrorist training camps near Herat, seizing control of the clandestine routes between Iran and Afghanistan. In his camps he allegedly dispensed his specialized knowledge of chemical weapons and poison to loyal acolytes, who were then dispersed to cities in Europe and the Middle East via a network of “people smuggling” operations. In 2001 he escaped to Iraq after suffering wounds to a leg during an American missile strike on his Afghan base. Some commentators claim his leg had to be amputated in Baghdad.

After September 11, 2001, German police in Hamburg uncovered a terrorist cell called al-Tawid, comprising Palestinian Arab activists trained in Zarqawi’s camps in Afghanistan. A man said to have been Zarqawi’s top aide, a 26-year-old Moroccan named Amer el-Azizi, planned the Madrid bombings in 2004 and also was allegedly involved in the planning for the 2001 attacks. However, the extent of Zarqawi’s involvement in these events is disputed.

American officials allege that, at Al Qaeda’s behest, Zarqawi established links with Ansar al-Islam, a group of militant Kurdish Muslims from northern Iraq. When the American aid official Laurence Foley was gunned down in Amman in October 2002, the Jordanians claimed that Zarqawi had planned and financed the atrocity.

Following the invasion of Iraq, Zarqawi stepped up his campaigns and is alleged to have been connected with a series of lethal bombings from Casablanca to Turkey, as well as more killings in Iraq. In late 2003 he began to emerge as the leader of the group Tawhid and Jihad (“Monotheism and Holy War”).

In August more than 50 people died when members of the group assassinated the moderate Shiite cleric Ayatollah al-Hakim in Najaf. Zarqawi also was blamed for organizing the suicide attack the same month on the U.N. headquarters in Baghdad that killed the U.N. envoy Sergio Vieira de Mello and forced the organization to pull out of Iraq.

In February 2004, the coalition released an intercepted letter, allegedly from Zarqawi, calling for attacks on Shiite targets to foment civil war. Within days bomb attacks on recruitment centers for the Iraqi armed forces had killed more than 100 people. The incident that brought him most notoriety, however, and which provoked the Americans to raise the price on his head to $25 million from $10 million, was the release of a videotape depicting the execution in May of 26-year-old Nicholas Berg, a businessman from Pennsylvania.

This atrocity, carried out with a machete, was allegedly committed by Zarqawi himself. “Where is the protection for Muslims’ pride in crusaders’ jails?” the executioner demanded to know. In the same month, Zarqawi’s group took the blame for killing the head of the Iraqi Interim Governing Council, and a few weeks later decapitated a South Korean civilian.

Some analysts saw Zarqawi as merely a figurehead around whom dissident groups in Iraq were rallying, rather than as an elusive fighter directing military operations. Certainly it seemed that the more the Americans blamed Zarqawi for terrorist atrocities, the greater his credibility on the Arab street. Dozens of guerrillas captured in the center of Baqubah in June 2004 were wearing headbands showing their loyalty to Zarqawi, the first time his supporters had been seen in public.

Unlike Mr. bin Laden, Zarqawi taunted his enemies with frequently released videotapes, statements, and commentaries broadcast through the Internet and Arab television stations. However, not all Iraqi dissidents supported Zarqawi. In early July 2004, a group calling itself Seif Allah accused Zarqawi and his group of treason and affiliation to Saddam’s discredited regime, and vowed to hunt them down. Last year, an unnamed Sunni insurgent leader was quoted as describing Zarqawi as “an American, Israeli and Iranian agent who is trying to keep our country unstable so that the Sunnis will keep facing occupation.”

The American military claimed to have injured Zarqawi in an assault in 2005; a statement released by Al Qaeda appeared to confirm this but said the injuries were minor. Whatever the truth, it made little difference to his dedication to violence. In the past year, he claimed responsibility for several attacks, including a triple suicide bombing in the Jordanian capital of Amman that killed 63 people, many of them guests at a wedding, and for a rocket attack from Lebanon into northern Israel.

Zarqawi’s father died in the 1980s. His mother, Umm Sayel, not a major influence on his life, died in 2004. His estranged first wife, Umm Muhammed, their four children, and his seven sisters and two brothers claimed in 2004 that they had not heard from him for five years.

Zarqawi’s second marriage is believed to have been childless.