A photo of Abu Bakr al-Baghdadi from US Army files relating to his time as a detainee in Iraq. Business Insider The world knows little of the Islamic State terror group's brutal leader, Abu Bakr al-Baghdadi, but a new article from counterterrorism expert Will McCants provides one of the most extensive accounts yet of his background.

McCants, director of the Project on US Relations with the Islamic World at the Brookings Institution, wrote an upcoming book on the Islamic State — aka ISIS, ISIL, and Daesh — and researched Baghdadi's life to explain his rise to become one of the most wanted terrorists in the world.

Since Baghdadi became the self-proclaimed "caliph" of ISIS in 2014, he has only appeared in public once, at a mosque in Mosul, Iraq. He was rumored to have died in an air strike earlier this year, but ISIS subsequently released a statement from him along with proof that he was still alive.

Even with new information about his life tricking out in the press, Baghdadi — aka Ibrahim Awwad Ibrahim Al-Badri — remains a mysterious and reclusive figure.

Here's what we know now about his background, as laid out by McCants in his Brookings essay:

Baghdadi was raised in a lower-middle-class family in Iraq. His relatives claimed to be descendants of the Prophet Muhammad.

His father taught at a mosque. When Baghdadi was a teenager, he led neighborhood children in Quran recitations.

Baghdadi's family had ties to late Iraqi dictator Saddam Hussein's Baath Party. Two of his uncles were involved with Saddam's security services, and two of his brothers served in the military under Saddam. One died during the Iran-Iraq war in the 1980s.

Members of Baghdadi's family were also thought to be Salafis, who follow a strict form of Islam that has been associated with ISIS' extreme interpretation.

Baghdadi was thought of as a quiet type, but when he read the Quran, his "quiet voice would come to life" and he would pronounce "the letters in firm, reverberating tones," according to McCants.

Samarra, Iraq, where Baghdadi was born. Getty Images

He was also known for having a temper. Once, when he saw women and men dancing together at a wedding, he got upset and forced them to stop.

Even in his youth, Baghdadi developed a reputation for being pious and following a strict interpretation of Islam. His nickname was "The Believer," and one of his brothers told McCants that Baghdadi "was quick to admonish anyone who strayed from the strictures of Islamic law."

Baghdadi wasn't a strong student in high school, but he went on to earn a doctorate degree in Quranic studies. He reportedly wanted to study law for his undergraduate degree, but his grades weren't good enough, so he studied the Quran instead.

He became a member of the Muslim Brotherhood, a group that seeks to establish Islamic states across the Middle East, but his views were more extreme than those of many of the others in the group. Baghdadi was reportedly drawn to the extremists, including his older brother, who wanted to overthrow un-Islamic rulers.

Still image taken from a video of a man purported to be Abu Bakr al-Baghdadi, the reclusive leader of the militant Islamic State, making what would be his first public appearance at a mosque in Mosul, Iraq. Thomson Reuters

Outside his religious studies, Baghdadi was fond of soccer. He was the star of a soccer club at a mosque at which he taught, and people compared him to the famous Argentinian player Lionel Messi. (This fits with an interview published earlier this year with a man who said he knew Baghdadi before he became ISIS' "caliph.")

Baghdadi is thought to have two wives and six children. McCants reports that the caliph's first wife, Asma, was the daughter of Baghdadi's maternal uncle.

He was initially involved with al-Qaida, which sent him to Syria after he was released from his detainment at the US-run Camp Bucca in Iraq in the early 2000s. There, he was tasked with "ensuring that AQI’s online propaganda was in line with its brand of ultraconservative Islam," according to McCants. Today, ISIS is known for its online propaganda that's highly effective at recruiting young people to join the terror group.

After ISIS broke away from al-Qaida, he was put in charge of religious affairs in some areas of Iraq. He became valuable to ISIS because the group needed religious scholars to establish legitimacy.

Reuters

This telling of Baghdadi's background suggests that his radicalization began long before he was imprisoned at Camp Bucca in the early 2000s. Although he was captured as a "civilian detainee" while he was visiting a friend who was wanted by American authorities, it's clear Baghdadi had already begun forming his extremist ideology by this point.

These details water down the notion that Baghdadi was radicalized while in American detention.

And Baghdadi likely knew what he was doing.

"For the ten months he remained in custody, Baghdadi hid his militancy and devoted himself to religious instruction," McCants wrote.

He was also able to meet and befriend ex-Baathists who would later join him in ISIS. The group's leadership is now thought to be made up largely of former Saddam loyalists, but that doesn't mean Baghdadi isn't devout or that he's just a religious figurehead for the organization.

McCants concluded: "The bare facts of Baghdadi's biography show an unusually capable man. ... Although the New York Times recently reported that he himself is making arrangements for a succession in the event of his demise by devolving many of his military powers to subordinates, his blend of religious scholarship and political cunning won't be easily replaced."