suggests that whoever released him from US custody four years ago may now be regretting it, the Telegraph Newspaper said in news briefed by “Shafaq News”.

Taken during his years as a detainee at the US-run Camp Bucca in southern Iraq, this is the only known photograph of Abu Bakr al-Baghdadi, the new leader of al-Qaeda in Iraq and Syria. But while he may lack the photogenic qualities of his hero, Osama bin Laden, he is fast becoming the new poster-boy for the global jihadist movement.

Well-organised and utterly ruthless, the ex-preacher is the driving force behind al-Qaeda’s resurgence throughout Syria and Iraq, putting it at the forefront of the war to topple President Bashar al-Assad and starting a fresh campaign of mayhem against the Western-backed government in Baghdad.

Last week, his forces fought open clashes with Iraqi army troops around the city of Fallujah - once known as the graveyard of the Americans - after brazenly attempting to seize control there the weekend before, the newspaper said.

“They turned up in convoys waving their black flags and saying that Fallujah belongs to al-Qaeda again,” said Ayad Dulaimi, a local resident. “With God’s help, the army will destroy them.”

For Washington, the fact that it is now Iraqi troops who are confronting Baghdadi’s fanatics rather than American ones is of limited comfort. For just like bin Laden, whose death he has vowed to avenge, his ambitions go well beyond the Middle East.

“You will see the mujahideen (holy warriors) at the heart of your country,” he warned the US in an audio-taped statement. “Our war with you has only started now.”

His attempted take-over of Fallujah and neighbouring Ramadi has also sparked bitter exchanges in Washington over the legacy of the Iraq war. As President Barack Obama flew back from his Christmas break in Hawaii last week, he faced accusations that he had squandered US sacrifices in Iraq through his decision to withdraw troops two years ago.

The charge was led by Senator John McCain, the 2008 Republican presidential candidate, who pointed out that hundreds of US troops had died during operations to keep Fallujah free of al-Qaeda. “Now we see people driving around Fallujah with black flags,” Senator McCain said. “It’s a disgrace.”

So who is exactly is the man who now so worries America, and why has he become so effective?

As with many of al-Qaeda’s leaders, precise details are sketchy. His FBI rap sheet offers little beyond the fact that he is aged around 42, and was born as Ibrahim Ali al-Badri in the city of Samarrah, which lies on a palm-lined bend in the Tigris north of Baghdad. Abu Bakr al-Baghdadi is a nom de guerre, as is his other name, Abu Duaa, which translates roughly as “Father of the Summons”.

Some describe him as a farmer who was arrested by US forces during a mass sweep in 2005, who then became radicalised at Camp Bucca, where many al-Qaeda commanders were held. Others, though, believe he was a radical even during the largely secular era of Saddam Hussein, and became a prominent al-Qaeda player very shortly after the US invasion.

“This guy was a Salafi (a follower of a fundamentalist brand of Islam), and Saddam’s regime would have kept a close eye on him,” said Dr Michael Knights, an Iraq expert at the Washington Institute for Near East Policy.

“He was also in Camp Bucca for several years, which suggests he was already considered a serious threat when he went in there.”

That theory seems backed by US intelligence reports from 2005, which describe him as al-Qaeda’s point man in Qaim, a fly-blown town in Iraq’s western desert, the newspaper added

“Abu Duaa was connected to the intimidation, torture and murder of local civilians in Qaim”, says a Pentagon document. “He would kidnap individuals or entire families, accuse them, pronounce sentence and then publicly execute them.”

Why such a ferocious individual was deemed fit for release in 2009 is not known. One possible explanation is that he was one of thousands of suspected insurgents granted amnesty as the US began its draw down in Iraq. Another, though, is that rather like Keyser Söze, the enigmatic crimelord in the film The Usual Suspects, he may actually be several different people.

“We either arrested or killed a man of that name about half a dozen times, he is like a wraith who keeps reappearing, and I am not sure where fact and fiction meet,” said Lieutenant-General Sir Graeme Lamb, a former British special forces commander who helped US efforts against al-Qaeda in Iraq. “There are those who want to promote the idea that this man is invincible, when it may actually be several people using the same nom de guerre.”

What does seem clear, however, is that al-Qaeda now has its most formidable leadership since Abu Musab al-Zarqawi, the Jordanian who kidnapped the British hostage, Ken Bigley, and who died in a missile strike in 2006.

When al-Baghdadi was announced as a new leader in 2010 - following the killing of two other top commanders - al-Qaeda was seriously on the back foot, not just in Iraq but regionwide. In former strongholds like Fallujah, its fighters had been routed after their brutality sparked a rebellion by local tribes. In Pakistan and Afghanistan, drone strikes were destroying the cream of its senior leadership. And the following year, the onset of the Arab Spring revolutions, with their emphasis on democracy and human rights, made it look simply irrelevant.

Indeed, when bin Laden himself was killed in May 2011, Baghdadi’s pledge to revenge his death with 100 terrorist attacks across Iraq looked like little more than bluster.

Today, he is already well past that target, thanks to a devastating campaign of car bombings and Mumbai-style killing sprees that has pushed Iraq’s death toll back up to around 1,000 per month.

“Baghdadi is actually more capable than the man he took over from,” said Dr Knights. “It’s one of those unfortunate situations where taking out the previous leadership has made things worse, not better.”

Quietly-spoken and publicity-shy, Baghdadi is said to be fond of turning up on frontline operations himself. Mindful, though, of the price on his head — second only to the $25m reward for al-Qaeda’s No 1, Ayman al Zawahari - he takes extensive precautions.

Fighters who have met him speak of a shadowy figure who can mimic a number of regional accents to blend in. In the company of all but the closest devotees, he wears a mask to prevent anyone getting a close look at him.

He has, however, won respect for being less gung-ho than other al-Qaeda leaders: while suicide bombers are a key part of his arsenal, he is said often to veto operations that put his other fighters at too much risk.

In the same spirit, his greatest coup so far was to free some of his most loyal supporters during a spectacular jail break at Baghdad’s Abu Ghraib prison, supposedly the most-heavily guarded facility in the country. Last July, a combined assault involving suicide bombers and 50 crack militiamen saw around 1,000 prisoners freed, half of them al-Qaeda members.

Many are believed to have headed to neighbouring Syria, where they have proved decisive in turning al-Qaeda into the pre-eminent rebel movement in the fight against President Assad.

Al-Baghdadi himself is also believed to have relocated there, and last year renamed his group the Islamic State of Iraq and the Levant (ISIS), which sees both countries as a single al-Qaeda caliphate. Already the group has about 7,000 fighters in northern Syria, including volunteers from Britain and Europe who may one day start terror campaigns at home.

Yet Baghdadi’s call for “all Muslims” to join him does not extend to Shias, whom al-Qaeda sees as apostates. Instead, his aim is to destroy President Assad’s minority Shia government in Syria, and then wrest control of Sunni-dominated western Iraq from the Shia-majority government in Baghdad, the newspaper concluded.