The insurgency waged by Boko Haram's leader Abubakar Muhammad Shekau, who claimed the kidnapping of more than 200 Nigerian schoolgirls, has grown so ruthless that even former Islamist allies have cut ties.
Born in a village in Nigeria's northeastern Yobe state on the border with Niger, Shekau had a traditional Islamic education in neighbouring Borno state, where Boko Haram was founded more than a decade ago by the cleric Mohammed Yusuf.
After meeting Yusuf, Shekau joined his movement made up largely of radical youths who believed that the prevalence of Western education and values were to blame for many of Nigeria's problems, including egregious corruption and crippling poverty.
Boko Haram, which loosely translates as "Western education is forbidden", is a nickname that the Islamists have disowned, referring to themselves as Jama'tu Ahlis Sunna Lidda'awati wal-Jihad (People Committed to the Propagation of the Prophet's Teachings and Jihad).
Awareness and condemnation of Shekau spread across the globe this week after he released a video boasting about the April 14 mass abduction in Chibok, Borno state, in which he threatened to sell the hostages as "slaves".
But for Nigerians, the chilling video was consistent with an Islamist leader who is believed to have masterminded waves of horrific attacks since he took charge of Boko Haram several months after Yusuf was killed by Nigerian police in 2009.
"With Shekau at the helm," the International Crisis Group (ICG) said in a report last month, "Boko Haram has grown more ruthless, violent and destructive."
Shekau's extremism is perhaps best highlighted by the decision of Ansaru — a Boko Haram offshoot which has kidnapped foreigners and published their execution online — to cut ties.
Ansaru "distanced itself from the rest of Boko Haram because it disapproved of its indiscriminate killings and Shekau's lack of tact," the ICG report said, citing security sources and people with close ties to both militant groups.
There were significant outbursts of violence under Yusuf but the group was nominally committed to spreading sharia (Islamic law) across northern Nigeria, a goal some in the deeply conservative region support.
Yusuf's ideology and anti-corruption preachings have been largely buried by Shekau's repeated attacks on defenceless civilians, including mass kidnappings and the slaughter of scores of students in their sleep, analysts say.
Even before Yusuf's death, Shekau had accused him of "being too soft", according to the ICG, and Shekau signalled the new direction he meant to take Boko Haram roughly a year after taking charge.
Major attacks in Nigeria's capital Abuja in 2012, including a bombing at the United Nations headquarters that killed scores, raised concern that Boko Haram's new leaders had received jihadist training abroad, perhaps in Algeria or Somalia.
The specific details of those foreign links have been much debated by experts but little has been confirmed.
Since 2011, the Islamists have attacked churches, mosques, politicians, police and the military, among various other targets.
The United States has declared Shekau a global terrorist and put a $7 million (5.3 million euros) bounty on his head.
The US Justice Department lists 1965, 1969 and 1975 as possible years of the birth.
And Shekau's videos have become the primary channel through which the insurgents speak.
At times he makes threats against specific Nigerian targets.
At others he seems completely disconnected from current events, threatening world leaders who are dead, like recent warnings against ex-British prime minister Margaret Thatcher, and the late pope John Paul II.
A quote from one his first video, released in 2012, has been cited by experts as perhaps providing a window into his character.
"I enjoy killing anyone that God commands me to kill the way I enjoy killing chickens and rams," Shekau said.
