Abu Musab al-Zarqawi rose from the life of a street thug in Jordan to become the symbol of "holy war" in Iraq, masterminding the bloodiest suicide bombings of the insurgency, beheading hostages and helping push Iraq into a spiral of sectarian violence with vicious attacks against Shi'ites.
The immediate impact of the 39-year-old terror leader's death on the insurgency was unclear, and his followers immediately vowed to continue their fight.
The Jordanian-born al-Zarqawi was the most prominent of the insurgency's leaders, but his strength within the movement was never certain. Homegrown Sunni Iraqi guerrillas - who are believed to have tense relations with al-Zarqawi - are thought to have had an equal or even greater role in deadly attacks on US and Iraqi forces and Shi'ites.
Still, al-Zarqawi was clearly instrumental in turning the swift US invasion of Iraq in 2003 into a grueling counter-insurgency fight. He became the symbol of the jihadi movement, nicknamed the "slaughtering sheikh" by his supporters across the Arab world.
Al-Zarqawi is believed to have personally beheaded at least two American hostages, Nicholas Berg in April 2004 and Eugene Armstrong in September 2004. Grisly videos of the slayings were posted on the Internet, part of a revolutionary Web propaganda campaign that was key to al-Zarqawi's movement.
Al-Zarqawi vowed fealty to al-Qaida leader Osama bin Laden in October 2004 and had the same bounty on his head from the US military - $25 million - as bin Laden.
But he played a dramatically different role: While bin Laden was the hidden leader, issuing statements from hiding in Pakistan's border region with Afghanistan, al-Zarqawi portrayed himself as the warrior on the front lines.
In the past year, al-Zarqawi moved his campaign beyond Iraq's borders, carrying out a November 9, 2005 triple suicide bombing against hotels in Amman that killed 60 people, as well as other attacks in Jordan and even a rocket attack from Lebanon into northern Israel.
He also sought to spread Sunni-Shi'ite strife across the Middle East. In an audiotape posted on the Web last week, he lectured Sunnis to stand up against Shi'ites and railed against Shi'ites for four hours, calling them enemies of Islam.
In April, he released a videotape showing his face for the first time in an apparent attempt to reinforce his image as the leader of Iraq's insurgents and a hero to Sunni extremists. The video emphasized dramatic, iconic images of al-Zarqawi, showing him in a desert landscape firing a machine gun.
The US military tried to undermine that image, issuing what it said were "outtakes" of that video captured in a raid, showing al-Zarqawi fumbling with the machine gun.
Born Ahmad Fadhil Nazzal al-Khalayleh on October 20, 1966, al-Zarqawi rose from a tough street life in the Jordanian industrial town of Zarqa - from which he eventually took his nom de guerre. He was one of 10 siblings in a poor branch of the prominent Bani Hassan Beduin tribe, which publicly renounced all ties to him following the triple hotel bombings in Amman.
In his teens, he was known as a thug, drinking alcohol and getting in street fights. He was jailed for six months for raping a girl, according to Jordanian security officials.
But he then embraced Islamic militancy, making his first trips to Afghanistan in the 1980s before returning to Jordan, where he was arrested in the mid-1990s.
It was in a Jordanian prison that he solidified his radical ideology. He shared a cell block with militant cleric Isam Muhammad al-Barqawi, known as Abu Muhammad al-Maqdisi, who became his spiritual mentor in "takfir" - an extremist strain of Islam that brands its enemies "kafirs" or "infidels" worthy of death.
After being released in an amnesty, al-Zarqawi went in 1999 to Afghanistan, where he formed links with bin Laden. He fled Afghanistan during the US-led war that ousted the Taliban in late 2001, passing through Iran to Iraq, according to US officials and militant biographies of al-Zarqawi posted on the Web.
His followers' first operations may have been in his homeland: Jordan has sentenced him to death in absentia for masterminding the October 2002 slaying of Laurence Foley, a diplomat and administrator of US aid programs in Jordan.
But soon after, his movement carried out two major suicide blasts in August 2003 - four months after Saddam's fall - that many see as marking the start of the insurgency in Iraq.
The first hit the UN headquarters in Baghdad that killed 22 people, including the top UN envoy to Iraq, Sergio Vieira de Mello, prompting the United Nations to pull its personnel out of the country.
The second targeted a Shi'ite shrine in Najaf that killed more than 85 people, including Shi'ite leader Ayatollah Muhammad Baqir al-Hakim.
For more than two years, al-Zarqawi wreaked mayhem across Iraq, his strikes targeted to inflict maximum casualties but also to weaken Iraq's fledgling security forces, cause political damage, or enflame sectarian violence.
His group claimed responsibility for the bloodiest single attack of the insurgency, a February 2005 suicide bombing against Iraqi security recruits in Hillah that killed 125 people.
His fighters are believed to be behind a string of suicide bombings against Shi'ites in Karbala and a police station north of Baghdad on January 5, 2006, that killed at least 130 people - only weeks after a landmark parliament election.
On May 18, 2004, a car bomb detonated by al-Zarqawi's fighters assassinated the president of the now disbanded Iraqi Governing Council, Izzadine Saleem.
The string of kidnappings of Westerners by his followers terrorized foreign workers in Iraq, forcing them to limit movements and take up costly security precautions.
Among the other hostage slayings claimed by al-Qaida in Iraq were American Jack Hensley, British engineer Kenneth Bigley, Kim Sun-il of South Korea and Shosei Koda of Japan, whose decapitated body was found dumped and wrapped in an American flag.
Al-Qaida in Iraq also kidnapped and killed the top Egyptian diplomat in Iraq and two Algerian diplomats, part of a campaign aimed at scaring Arab nations from sending full ambassadors to Baghdad in support of the new, Shi'ite-led government.
But in the last months of his life, there were signs al-Zarqawi's attacks on civilians were eroding his support. The triple hotel bombing in Amman - which killed mostly Sunnis - outraged many in Jordan and even brought criticism from other Islamic militants.
In January, al-Zarqawi announced that his group was joining a larger umbrella group of Iraqi insurgents called the Shura Council of Mujahedeen. This was seen as an attempt to give an Iraqi face to his group, which was believed to be mainly made up of non-Iraqi, Arab fighters.