Baghdadi's FBI rap sheet offers little beyond the fact that he is 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 and 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.
US intelligence reports from 2005 describe him as al-Qaeda's point man in Qaim, a fly-blown town in Iraq's western desert.
"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 he 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 drawdown in Iraq. Another, though, is that 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."
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 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 1000 a month.
His greatest coup so far was to free some of his most loyal supporters during a spectacular jailbreak at Baghdad's Abu Ghraib prison. Last July, a combined assault involving suicide bombers and 50 crack militiamen freed around 1000 prisoners, half of them al-Qaeda members.
Many are believed to have headed to Syria, where they have proved decisive in turning al-Qaeda into the pre-eminent rebel movement in the fight against Assad.
Al-Baghdadi himself is also believed to have relocated there, and last year renamed his group the Islamic State of Iraq and Syria (ISIS), which sees both countries as a single al-Qaeda caliphate. The group has about 7000 fighters in northern Syria, now engaged in fighting other rebel groups.
"Al-Qaeda may be better organised under Baghdadi," said Dr Toby Dodge, an Iraq expert at the London School of Economics. "But as soon they hold territory, their popularity tends to disappear."
