Abubakar Shekau, the leader of Nigeria’s Islamic extremist group Boko Haram, is claiming responsibility for the abduction of 300 teenage schoolgirls and is threatening to sell them “in the marketplace.”

The chilling proclamation drew worldwide attention Monday, but Shekau has been a notorious figure in Nigeria for years. Still, little is known about his rise to the top in Boko Haram, which loosely translates to: “Western education is forbidden” (or “sinful”).

Who is Abubakar Shekau?

Shekau was reportedly Boko Haram’s deputy leader until its founder, Mohammed Yusuf, was killed in 2009. Nigerian authorities believed that Shekau was also killed in 2009, but he later surfaced in a video claiming to be Boko Haram’s new leader.

The group is considered to be a terrorist organization that aims to overthrow the Nigerian government and install a regime based on radical Islamic views.

Shekau’s exact age is unknown. He was reported to be in his mid-30s, while some have said that he is 43. According to a BBC profile published last year, Shekau has been described as a “fearless loner” with a photographic memory. He is influenced by radical ideology, once proclaiming that he enjoys killing “anyone that God commands me to kill.”

According to the U.S. Department of State, Shekau has expressed solidarity with al Qaeda and threatened the U.S.

Who is looking for him?

The Nigerian authorities have blamed numerous bombings and other crimes on the followers of Boko Haram. They have long speculated that Shekau had been killed in fighting with security forces, but that doesn’t appear to be the case.

In 2012, the U.S. Department of State declared Shekau a “specially designated global terrorist.”

The following year, the department offered a reward of up to $7 million for information leading to Shekau’s location. The U.S. government said that Boko Haram is responsible for a 2011 bomb attack on a United Nations building in Abuja, Nigeria, which killed 23 people and injured dozens more.

Why did Shekau kidnap the schoolgirls?

In a video reviewed by The Associated Press, Shekau claims he abducted the girls so they can remain “slaves” with Boko Haram. He appears to be making a reference to the ancient jihadi custom of enslaving women captured in a holy war.

"By Allah, I will sell them in the marketplace,” Shekau is reportedly heard saying in the video in the Hausa language.

“They are slaves and I will sell them because I have the market to sell them."