If US intelligence is to be believed, Abu Musab al-Zarqawi is the ultimate bad guy.
Dangerous, ruthless and elusive, he is one of the most wanted men in the world.
Washington has linked al-Zarqawi to al-Qaida and says he is the prime suspect in some of the deadliest attacks of recent years.
More specifically, US authorities accuse the 38-year-old Jordanian of masterminding a string of spectacular bombings in Iraq.
They even increased the bounty on his head on Thursday to $25 million - the same as al-Qaida chief Usama bin Ladin.
But despite the allegations, little is known about the man himself - and some experts even doubt he is alive.
Others, meanwhile, have accused the Americans of exaggerating the threat he poses to discredit resistance to their occupation of Iraq.
Known to have been born in the suburbs of the Jordanian capital Amman, information about al-Zarqawi's early years is scarce apart from his reputed "piety".
But what is sure is that around 1990 he travelled to Afghanistan to join other Arabs to fight pro-Soviet forces in the country.
Experts speculate it was here that al-Zarqawi imbibed the "Afghan-Arab" spirit of pan-Islamism and the conviction that all Arab regimes were corrupt and un-Islamic.
"Al-Zarqawi has become ... the perfect bogeyman... He is a foreigner, so it is the perfect way for the Americans to discredit the resistance and
say these attacks are
not coming from the
Abd al-Bari Atwan,
Arab affairs commentator
Paul Rogers, of the UK-based Bradford University's peace studies department, says groups influenced by the Afghan-Arab ideology have four main aims.
"These groups want to remove American troops from the Gulf," he told Aljazeera.net.
"They want to overthrow the monarchy in Saudi Arabia which they see as being illegitimate and unworthy of being custodians of the Islamic holy places.
"They also want a just solution to the Palestine issue, and have a longer-term goal of wanting to see the Arab regimes replaced by some sort of Caliphate."
After the Russians were defeated in Afghanistan, al-Zarqawi went back to Jordan but was soon arrested after being accused of conspiring to overthrow the monarchy and establish an Islamic state.
Jailed in Jordan
He spent at least four years in jail and, upon his release, fled the country. Shortly afterwards, Jordan sentenced him to death in absentia for plotting attacks on Israeli and American tourists.
At this point it is reckoned that al-Zarqawi returned to Afghanistan, where he had several meetings with Usama bin Ladin and fought on his side during the US-led invasion of the country in 2001.
Was Ayat Allah al-Hakim an Al-
Zarqawi victim? Yes, says the US
It is also alleged he set up a training camp in the western city of Herat, near the border with Iran. Students at his camp reportedly became experts in the manufacture and use of poison gases.
Al-Zarqawi is then believed to have fled to Iraq in 2001 after losing a leg in a US missile strike on his Afghan base.
US officials argue it was at al-Qaida's behest that he moved to Iraq and established links with Ansar al-Islam - a group of Kurdish Islamists.
There is even speculation that al-Zarqawi was killed during a US attack on an Ansar al-Islam base last year.
Arab commentator Abd al-Bari Atwan says it is quite possible that al-Zarqawi is now dead.
He told Aljazeera.net: "There is no real proof that he is alive. If he is supposedly moving around freely in Iraq, why haven't Iraqis spoken about him? He can't be that difficult to recognise with his wooden leg."
The rumours of al-Zarqawi's demise did not stop the Jordanian authorities, though, from accusing him of masterminding and financing the killing of a US official in Amman in October 2002.
"If the Americans capture al-Zarqawi, they can claim a great victory and say they have knocked off the head of the organisation and thus rendered the whole network ineffective"
Bradford University, UK
He was also named as the brain behind a series of lethal bombings in 2003 from Morocco to Turkey.
But it is in Iraq that al-Zarqawi is alleged to have been most active.
In the run-up to the Iraq war in February 2003, US Secretary of State Colin Powell told the United Nations that al-Zarqawi was an associate of Usama bin Ladin who had sought refuge in Iraq.
According to Powell, this was a sure sign that Saddam Hussein was courting al-Qaida which, in turn, justified an attack on Iraq.
Once again, Bradford University's Rogers casts doubt on the US allegation.
"Al-Zarqawi is probably a pretty significant figure but he is being made into this Usama bin Ladin-type symbol, which seems to be a natural thing for the Americans to do. It seems they like to focus on personalities," Rogers said.
Abd al-Bari Atwan agrees.
"Al-Zarqawi has become the new superpower, the perfect bogeyman. The Americans are just building him up to mask their failure in Iraq and their inability to maintain law and order.
The US says al-Zarqawi has links
with Usama bin Ladin's al-Qaida
"He is a foreigner, so it is the perfect way for them to discredit the resistance and say these attacks are not coming from the Iraqi people."
Since last year's US-led invasion of Iraq, al-Zarqawi has been accused of a string of deadly attacks in the country.
The assassination of the Shia cleric Ayat Allah Baqir al-Hakim in Najaf was one of the bloodiest attacks in Iraq last year - more than 80 Shia worshippers died. US authorities pinned the blame on al-Zarqawi.
And in May he is alleged to have been involved in the beheading of a US contractor, Nick Berg, which was shown on a video.
A $5 million bounty on al-Zarqawi's head was doubled after US authorities intercepted a letter that they claimed confirmed he was working with al-Qaida to drive the US out of Iraq.
In it the author accused Iraqi Shias of collaborating with foreign invaders and called for a sectarian conflict in Iraq as a means of undermining the US presence there.
The author also claimed to have already undertaken 25 successful attacks against the enemy.
Arab affairs commentator Abd al-Bari Atwan does not believe the letter was genuine.
"I personally think that letter was the product of the intelligence services. Don't forget it was [new Iraqi interim prime minister] Iyad Allawi who distributed the letter to the press.
"There is no real proof that he [al-Zarqawi] is alive. If he is supposedly moving around freely in Iraq why haven't Iraqis spoken about him? He can't be that difficult to recognise with his wooden leg"
Add al-Bari Atwan,
Arab affairs commentator
"Al-Zarqawi would have had to have been really stupid to send a letter like that which promoted civil war in Iraq."
Bradford University's Rogers says Washington's fixation with al-Zarqawi is a symptom of its misunderstanding of the threats that it faces.
"The Americans like to have a known enemy and they can't think in terns of a coalition and a network which is diffuse.
"If they capture al-Zarqawi, they can claim a great victory and say they have knocked off the head of the organisation and thus rendered the whole network ineffective."
But he added: "These tactics are seriously flawed. Several studies have proved that support for al-Qaida and related groups has increased in the last three years because the amount of attacks have gone up. The US just can't fight their so-called war on terror like this."