Abu Musab al-Zarqawi is Iraq's most notorious insurgent - a shadowy figure associated with spectacular bombings, assassinations and the beheading of foreign hostages.
He first appeared in Iraq as the leader of the Tawhid and Jihad insurgent group, merging it in late 2004 with Osama Bin Laden's al-Qaeda network.
Recent information on Zarqawi is restricted to what his enemies and supporters have attributed to him.
While many analysts say he is using the Iraqi insurgency as a springboard to expand his operations, others argue his influence has been exaggerated.
Exporting the insurgency
The video-taped beheadings of foreign hostages that made Tawhid and Jihad infamous in 2004 have become less frequent since the group's merger with al-Qaeda.
Bomb attacks on Iraq's Shia-dominated government and security forces have continued apace, however, with many of the bloodiest strikes of 2005 blamed on Zarqawi's group, now renamed al-Qaeda in Iraq.
According to BBC security correspondent Gordon Corera, Zarqawi's increasingly bloody attacks on the Shias are alienating many in the insurgency, including some Sunni Muslims who are its strongest backers.
A letter released by US forces in 2005 - allegedly authored by Bin Laden's deputy, Ayman al-Zawahiri, and addressed to Zarqawi - appears to support this.
In the letter, whose authenticity remains in doubt, Zawahiri purportedly cautions Zarqawi that indiscriminate attacks on the Shia are eroding support for al-Qaeda.
Our correspondent says Zarqawi may be shifting his focus away from Iraq, exploiting his foreign connections in order to export the violence further afield.
Bin Laden rival?
The US 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.
A 'wanted' poster for Zarqawi: there is $25m bounty on his head
Several men alleged to be key aides of Zarqawi have also been killed or captured - but these appear to have had no effect on his group's ability to operate.
The US is offering a $25m bounty on Zarqawi's head - the same sum they are offering for Bin Laden himself.
The reward was increased in early 2004, 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.
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 at the time 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.
Ayatollah al-Hakim's death was a blow to Iraqi Shias
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.
In October 2002, Zarqawi was blamed for the assassination of US aid official Laurence Foley in Amman.
Months later, in 2003, he was named as the brains behind a series of lethal bombings - from Casablanca in Morocco to Istanbul in Turkey.
It is in Iraq, though, that he has been most active.
An intercepted letter released by the Americans in February 2004 seems to support their claim that targeting Shias is central to Zarqawi's strategy in Iraq.
In it, Zarqawi appears 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 are now a daily occurrence in Iraq. 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.