No one knows how old he is. Some say 35. Some say 36. Others think he’s 44. Twice he was believed dead, and twice he reemerged to conduct an even broader campaign of killing and terror that made him one of the most wanted men in the world.

His name is Abubakar Shekau. He is the leader of Boko Haram. And he has your girls.

“I abducted the girls at a Western education school,” Shekau proclaimed on Monday in a video, clutching a rifle among several masked men. “And you are disturbed. I said Western education should end. … I abducted your girls. I will sell them in the market, by Allah. There is a market for selling humans. Allah says I should sell; he commands me to sell. I will sell women. I sell women.”

Shekau, who has a $7 million bounty on his head, grinned a mouth of white teeth. His face was patched by scruff. He raised his arm as though delivering a sermon — and to Shekau, who considers himself a devout holy man, he was. For a group as fragmented and diverse as the Boko Haram, which kidnapped hundreds of Nigerian school girls three weeks ago, one of the few unifying factors is extremist ideology. And no one believes in the cause more than Shekau, a complex, intensely private figure.

“It is Allah that instructed us,” Shekau said in the video released Monday. “Until we soak the ground of Nigeria with Christian blood and so-called Muslims contradicting Islam. After we have killed, killed, killed, and get fatigue and wondering what to do with their corpses — smelling of [Barack] Obama, [George] Bush and [Goodluck] Jonathan — will open prison and be imprison the rest. Infidels have no value.”

Where does such vengeance come from? What does he want? Who is he?

A review of academic and first-hand accounts reveal Shekau to be both an intellectualizing theologian and a ruthless killer. Raised Muslim, he was born sometime in the 1970s in a border town named Shekau between Niger and Nigeria — in the heart of the former Sokoto caliphate.

In 1990, he moved to a town that would become the birthplace of Boko Haram to study under a traditional cleric, according to the International Crisis Group. In the early 2000s, he met its future charismatic leader, Muhammad Yusuf. Shekau became one of his earliest acolytes, and was soon one of the top lieutenants in the group.

Intense and quiet, Shekau was more bookish than the group’s gregarious leader, Yusuf. “Shekau was always studying and writing, and was more devoted and modest than anyone else,” Ahmad Salkida, a man considered the Nigerian authority on Boko Haram, told the Financial Times in 2012. “He would only wear cheap clothes and did not accept even to drive a car, preferring a motorbike.”

Together, the men built what Salkida described in a separate account as an “imaginary state within a state.” Boko Haram was a sophisticated apparatus: a cabinet of leadership, a brigade of guards, a military branch, a large farm, and “an effective micro finance scheme.” It lured in the area’s impoverished and uneducated youths. “Boko Haram was founded on ideology, but poor governance was the catalyst for it to spread,” Salkida said. “If there had been proper governance and a functioning state, Yusuf would have found it very difficult to succeed.”

But even in those days, there was something disquieting about Shekau. “Even when Boko Haram was peaceful,” Salkida explained, “he was somehow more feared than Yusuf.”

Boko Haram, however, wouldn’t stay peaceful for long. Its clashes with Nigerian forces between 2004 and 2006 grew in intensity, and as the years ground past, Shekau became increasingly unmanageable. Yusuf “had trouble keeping his unruly lieutenants, particularly Shekau, in check,” reports the International Crisis Group.

In 2009, Yusuf was captured by the Nigerian authorities in a battle that appeared to kill Shekau as well. Yusuf was soon killed in prison, and Boko Haram, deprived of its chief, appeared on the verge of collapse. But then, less than a year later, and appointed the new leader because he was “radical and aggressive,” Shekau released a video, vowing to exterminate Western culture and education in Nigeria.

What he lacked in oratorical capabilities, he made up for in bellicosity. “I enjoy killing anyone that God commands me to kill,” he said after orchestrating an attack that claimed 180 lives. “The way I enjoy killing chickens and rams.”

Boko Haram has a similar operational structure to al-Qaeda. There are individual cells that affiliate under the same name, but operate autonomously. “A lot of those calling themselves leaders in the group do not even have contact with him,” Salkida told the BBC last year.

But even with such division, Shekau has maintained control — and created a mystique — through his brutality and ability to survive. In 2013, the Nigerian military again announced he had likely been killed. But he later surfaced once more in a fresh video, saying he was “protected by Allah.”

“Why is he so violent? I think because Shekau was almost killed,” Martin Ewi, a senior researcher at the Institute for Security Studies, told France 24. “Imagine coming back from the dead. He knows he doesn’t have a second chance if he’s caught by the security forces. … He was in the mouth of the crocodile, now he’s coming back to kill the crocodile.”