Profile: Abu Musab al-Zarqawi
Abu Musab al-Zarqawi
Zarqawi's Jordanian roots
Abu Musab al-Zarqawi - a man notorious for his alleged ruthlessness - is suspected of direct involvement in the kidnap and beheading of several foreigners in Iraq - even of wielding the knife himself.
Washington has also accused the 37-year-old Jordanian radical of masterminding a string of spectacular suicide bombings in Iraq, and of being linked to al-Qaeda.
After viewing a video of the beheading of American engineer Eugene Armstrong, taken hostage in Baghdad in September 2004 along with a fellow American and a Briton, the CIA believes with a "high degree of confidence" that it was Zarqawi who read out a statement and then carried out the murder.
The video followed a pattern which has become grimly familiar since American contractor Nick Berg was shown being killed in May 2004.
A group of militants clad in black stand in front of the banner of Zarqawi's group, Tawhid and Jihad, with their victim kneeling before them.
After reading a statement, a militant leans over the bound and blindfolded prisoner and cuts off his head with a knife.
Those killed in this fashion include another American, a South Korean and a Bulgarian. A Turkish hostage was shot three times in the head.
Bin Laden rival?
Zarqawi's network is considered the main source of kidnappings, bomb attacks and assassination attempts in Iraq.
Although he is thought to have links with al-Qaeda, experts regard his group as autonomous - perhaps even a rival to Osama Bin Laden's organisation.
The US has put a $25m bounty on his head - the same sum they are offering for Bin Laden himself.
The reward was increased after American authorities intercepted a letter which, they claimed, confirmed he was working with al-Qaeda to drive the US out of Iraq.
In the run-up to the Iraq war in February 2003, US Secretary of State Colin Powell told the United Nations Zarqawi was an associate of Osama Bin Laden who had sought refuge in Iraq.
A 'wanted' poster for Zarqawi: there is $25m bounty on his head
Intelligence reports indicated he was in Baghdad and - according to Mr Powell - this was a sure sign that Saddam Hussein was courting al-Qaeda, which, in turn, justified an attack on Iraq.
But some analysts contested the claim, pointing to Zarqawi's historical rivalry with Bin Laden.
Both men rose to prominence as "Afghan Arabs" - leading foreign fighters in the "jihad" against Soviet forces in Afghanistan in the 1980s.
It was a far cry from Zarqawi's youth as a petty criminal in Jordan, remembered by those who knew him as a simple, quick-tempered, and barely literate gangster.
But after the defeat of the Soviets in Afghanistan, Zarqawi went back to Jordan with a radical Islamist agenda.
Sentenced to death
He spent seven years in prison there, accused of conspiring to overthrow the monarchy and establish an Islamic caliphate.
Not long after his release, he fled the country.
Jordan tried him in absentia and sentenced him to death for allegedly plotting attacks on American and Israeli tourists.
Western intelligence indicated Zarqawi had sought refuge in Europe.
German security forces later uncovered a militant cell which claimed Zarqawi was its leader.
The cell-members also told their German interrogators their group was "especially for Jordanians who did not want to join al-Qaeda".
According to the German intelligence report, this "conflicts with... information" from America.
The next stop on his itinerary was his old stamping ground - Afghanistan.
He is believed to have set up a training camp in the western city of Herat, near the border with Iran.
Students at his camp supposedly became experts in the manufacture and use of poison gases.
It is during this period that Zarqawi is thought to have renewed his acquaintance with al-Qaeda.
He is believed to have fled to Iraq in 2001 after a US missile strike on his Afghan base, though the report that he lost a leg in the attack has not been verified.
US officials argue that it was at al-Qaeda's behest that he moved to Iraq and established links with Ansar al-Islam - a group of Kurdish Islamists from the north of the country.
He is thought to have remained with them for a while - feeling at home in mountainous northern Iraq.
When US aid official Laurence Foley was gunned down in Amman in October 2002, the Jordanian authorities claimed he had masterminded and financed the attack.
If the intelligence agencies are to be believed, it was just the beginning of a busy year for Zarqawi.
In 2003, he was named as the brains behind a series of lethal bombings - from Casablanca in Morocco to Istanbul in Turkey.
Later Spanish officials were reported to be looking into allegations that he may have been behind the Madrid bombings on 11 March 2004, which killed 191 people.
It is in Iraq, though, that he appears to be most active.
The assassination of the Shia cleric, Ayatollah al-Hakim, at a shrine in the town of Najaf, was one of the bloodiest attacks in Iraq last year - over 50 Shia worshippers died.
US authorities pinned the blame on Zarqawi.
The intercepted "Zarqawi" letter released by the Americans in February 2004 seems to support their claim.
In it, the author appeared to share his plans for igniting sectarian conflict in Iraq as a means of undermining the US presence there. And he claims to have already undertaken 25 successful attacks against the enemy.
Within days of the letter's release, bomb attacks on recruiting centres for the Iraqi security forces had killed nearly 100 people.
Attacks have continued across Iraq almost daily in recent months. Whether or not Zarqawi is behind them all, he is seen by the US as the biggest obstacle to their hopes of progress in Iraq - their most dangerous enemy in the country.
BBC NEWS | Middle East | Profile: Abu Musab al-Zarqawi
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Profile: Abu Musab al-Zarqawi