Several facts, however, are clear. Baghdadi leads the Islamic State in Iraq and the Levant (ISIL), the group that evolved from al-Qaeda in Iraq.

He appears to be a shrewd strategist, a prolific fundraiser and a ruthless killer, and the United States has a US$10 million bounty on his head.

He has thrown off the yoke of al-Qaeda's command and just took his biggest prize yet in Mosul, a city that sits at the key intersection of Iraq, Turkey and Syria. And in just one year of grisly killing, he has in all likelihood surpassed even al-Qaeda leader Ayman al-Zawahiri in power among Islamist militants.

"The true heir to Osama bin Laden may be Abu Bakr al-Baghdadi," wrote David Ignatius. He is "more violent, more virulent, more anti-American", a senior US intelligence official told Ignatius.

"For the past 10 years or more, [Zawahiri] has been holed up in the Afghanistan-Pakistan border area and hasn't really done very much more than issue a few statements and videos, whereas Baghdadi has done an amazing amount," said Richard Barrett, a former counterterrorism chief with the British foreign intelligence service. "He has captured cities, he has mobilised huge amounts of people, he is killing ruthlessly throughout Iraq and Syria … If you were a guy who wanted action, you would go with Baghdadi."

Born a Sunni in 1971 in the Iraqi city of Samarra with the name Awwad Ibrahim Ali al-Badri al-Samarrai, he claims to be a direct descendant of the Prophet Mohammed.

According to a widely cited biography released by jihadists, "he is a man from a religious family. His brothers and uncles include preachers and professors of Arabic language, rhetoric and logic."

The biography and Arabic-language accounts claim he obtained a doctorate at the former Islamic University in Baghdad, which is presumably why several of his many aliases include the title "Dr".

He holds degrees in Islamic studies and history and is thought to have been a preacher, aged about 30, around the time of the 2003 US-led invasion of Iraq. The chaos of those months drove him into militancy and he formed an armed group in eastern Iraq, one that reportedly never rose out of obscurity.

In 2005, Baghdadi was captured by US forces and he spent the next four years as a prisoner in Camp Bucca in southern Iraq. It was from his time there that the first known picture emerged.

He gained enough respect that by 2010, after several leaders of the insurgent group al-Qaeda in Iraq, including Abu Musab al-Zarqawi were killed, he assumed control.

At that time, the power of the Islamist militancy in Iraq was at its lowest ebb. But then Syria happened. The civil war there, which left a vacuum of authority in large tracts of the country, fuelled a resurgence of the group.

Sensing an opportunity, Baghdadi sent an aide across the border to Syria to expand al-Qaeda's foothold there. That aide, Abu Mohammad al-Golani, set up al-Qaeda's Nusra Front which quickly rose to prominence with a series of deadly car bombings.

But as Golani grew strong in Syria and rejected an edict to merge his forces under Baghdadi's command, Baghdadi launched a war against the Nusra Front, leading to a split with Zawahiri.

For many of Baghdadi's supporters the clash between the two was no surprise.

When bin Laden was killed in Pakistan three years ago, Baghdadi "was the only one who did not pledge allegiance to Zawahiri", a non-Syrian ISIL member said.

"He was assigned by Sheikh Osama to establish the state, this was his plan before he [bin Laden] was killed."

While Baghdadi's supporters believe an Islamic state would revive the glories of Islam under the Prophet Mohammed, they say Zawahiri feared that by drawing jihadi fighters together in one place it would make it easier for the West to defeat them.

His fighters counter that Baghdadi has plenty of hidden surprises for his enemies.

"He has capabilities that he keeps secret until the right time," another ISIL supporter said.

ISIL also drew the ire of many Syrian opposition fighters by focusing not on the fight against Assad, but rather on restoring a medieval Islamic state, or caliphate, in Iraq and greater Syria, also known as the Levant, a traditional name that refers to a region stretching from southern Turkey to Egypt on the eastern Mediterranean. The group is also referred to sometimes as the Islamic State of Iraq and Syria (Isis).

Ignoring Zawahiri's calls to leave Syria to the Nusra Front, Baghdadi expanded operations across northern and eastern parts of Syria in 2012 and last year, sometimes battling Assad's forces but more often pushing out other rebel fighters.

ISIL's unforgiving treatment of ordinary Syrians won it many enemies and by the end of last year an alliance of Nusra and other Islamist brigades struck back, pushing ISIL back to its stronghold along the Euphrates River in the oil-producing deserts of eastern Syria.

But ISIL has grown stronger, not weaker. Baghdadi's fighters control the city of Raqqa, Syria's only provincial capital beyond Assad's control, and have imposed strict Islamic law.

In neighbouring Deir al-Zor province ISIL has seized oilfields and towns on the northeast bank of the Euphrates, 100km from the Iraqi border. Oil sold on the black market provides millions of dollars in revenues, rebels say. Combined with Iraqi recruits and the military equipment seized in his capture of Mosul, Baghdadi now has a formidable array of resources.

Supporters say that is key to his aim of military self-sufficiency, ensuring an independent flow of money, manpower, weapons and energy supplies.

Baghdadi has also opened the door to foreign fighters, particularly Europeans and Americans, providing them with training and a sense of purpose. While they are useful on the Syrian battlefield, they may also return home as veterans with experience to recruit others to carry out attacks for Baghdadi outside the Middle East.

They are trained to be fearless and merciless. Activists in several areas inside Syria say that Baghdadi's men walk around wearing explosive vests.

Baghdadi is respected among militants as a battlefield tactician but still maintains his anonymity. He is not one for video-taped pronouncements. Some reports claim, perhaps fancifully, that he wears a mask when addressing his commanders, earning him the nickname "the invisible sheikh".

Asked how serious Baghdadi is, a supporter replied: "When you have his army, his determination and his belief then the world should fear you."

"If the world does not fear Baghdadi then they are fools, they do not know what will hit them in the future."