Boko Haram leader Abubakar Shekau: A ruthless leader with a twisted ideology

Shekau was born in Shekau village that borders Niger. He studied under a cleric and then attended Borno State College of Legal and Islamic Studies for higher studies on Islam.

That's why he's also known as 'Darul Tawheed,' which translates to an expert in monotheism, or the oneness of Allah.

He speaks several languages fluently: Hausa, Fulani, Kanuri and Arabic. But English isn't one of them. After all, he heads a group that rejects all things Western.

Even his age is unknown -- estimates range between 38 and 49.

The U.S. State Department has Shekau's year of birth listed as 1965, 1969 and 1975.

Analysts describe Shekau as a loner and a master of disguise. He does not speak directly with members, opting to communicate through a few select confidants.

He uses many aliases: Abu Bakr Skikwa, Imam Abu Bakr Shiku and Abu Muhammad Abu Bakr Bin Muhammad Al Shakwi Al Muslimi Bishku among them.

Boko Haram was founded by Mohammed Yusuf, a charismatic, well-educated cleric who drove a Mercedes as part of his push for a pure Islamic state in Nigeria. He wasn't too effective as a leader and had a hard time keeping his second-in-command in check. Shekau was more radical and had grander designs.

Mohammed Yusuf was killed in a security crackdown in 2009, along with about 700 of his followers. That left Shekau in charge. He vowed to strike back, and his group has spared no one: government workers, police officers, journalists, villagers, students and churchgoers. Human Rights Watch estimates that in the past five years, more than 3,000 people have been killed.

The Nigerian military has touted Shekau's death several times, only to retract its claim after he appeared alive and vibrant in propaganda videos.

They almost got him in September 2012 when they raided his home, where he had snuck in for his six-day-old baby's naming ceremony, according to the International Crisis Group. He managed to get away with a gunshot wound to the leg; his wife and three children were taken by the military.

The northeast, where Boko Haram has been most active, is economically depressed and among the least educated regions in Nigeria. Shekau has done a good job of convincing residents that the powers in Abuja are corrupt and a better system of government would be a strict enforcement of Islamic Sharia law across Nigeria. And his promise, coupled with a weapon and a license to plunder, has been enticing to hundreds of young men.

The central government's heavy-handed and frequently untargeted anti-terrorism campaign has just helped create more members to sustain Boko Haram. The country's own Human Rights Commission last year accused the military of arbitrary killings, torture and rape in its campaign against the group. This makes for fertile territory for Boko Haram.

There's no firm evidence as yet that Boko Haram has ambitions beyond Nigeria. But its campaign of terror has spilled into remote parts of Cameroon and it appears to have informal links with militant Islamist groups in Mali and Niger.

It was in May 2013 that Shekau first announced in a video that Boko Haram would start kidnapping girls. The kidnappings, he said, were retaliation for Nigerian security forces nabbing the wives and children of group members.

The most horrifying instance was last month's abduction of 276 girls from a girl's school.

"I abducted your girls," he taunted with a chilling smile in a new video that surfaced this week. "There is a market for selling humans. Allah says I should sell. He commands me to sell."

Shekau has been on the radar of U.S. officials since he came to power in 2009. Last June, the United States put a bounty on him, offering a reward of up to $7 million for information leading to his location.

Here's why, says CNN's Chief International Correspondent Christiane Amanpour: "(African warlord) Joseph Kony's had a bounty for years and years. Even with the 'Stop Kony' video that went viral, nothing has happened to get Joseph Kony -- even though it's about the only thing in Africa that the United States has committed some forces and some intelligence to.

"Osama bin Laden was not given up because of the $25 million bounty. And who knows whether this will be the case."