Boko Haram is an armed group blamed for killing hundreds of people in northern and central Nigeria. Many of the attacks have targeted places of worship, often churches, but Muslims have also been killed.
The group was founded in 2002 in Maiduguri, the capital of Borno state, allegedly by Mohammed Yusuf, a religious teacher.
In 2004, it moved to Kanamma in Yobe state, close to the border with Niger, where it set up a base dubbed "Afghanistan", from which it attacked nearby police outposts.
Boko Haram means "Western education is sin" in the Hausa language spoken in Nigeria's north. It is believed to have a number of factions with differing aims, including some with political links.
The group initially claimed to be fighting for the creation of an Islamic state in the north, but a range of demands by different people have since been issued.
Criminal gangs are also believed to have carried out violence under the guise of Boko Haram. Conspiracy theories abound as well, including whether enemies of President Goodluck Jonathan, a southern Christian who faces strong opposition in the north, have backed the violence.
|Attacks blamed on Boko Haram has
killed hundreds of people [Reuters]
The group's first real leader was the late Mohammed Yusuf, who convinced young people to join him despite having only elementary knowledge of the Quran, according to one professor who has studied the group.
His former deputy Abubakar Shekau is widely believed to currently lead Boko Haram's main cell, which says it wants to be known by a different name, roughly translated as "People Committed to the Prophet's Teachings for Propagation and Jihad".
Boko Haram launched an uprising in 2009, leading to nearly a week of fighting that ended with a military assault which left about 800 dead and the group's mosque and headquarters in northeastern Maiduguri in ruins.
Yusuf was captured and later killed when police said he was trying to escape from custody - but his body was found in the street, still handcuffed, raising concerns that he had been the victim of an extrajudicial killing.
Boko Haram went dormant for more than a year before re-emerging in 2010 with a series of assassinations. Bomb blasts, including suicide attacks, have since become frequent and increasingly deadly.
Salisu Mohammed, a conflict management specialist, told Al Jazeera that Nigerian authorities should have acted sooner to stop the proliferation of Boko Haram.
"Many people have known of the existence of this group, silently and within the community, especially in the last year," he said. "They are becoming more extreme because in the past there wasn't a major push in place to check their proliferation."
In May of 2013, the Nigerian army did launch a major offensive against the group, deploying thousands of troops in the north and launching aerial bombardments of suspected Boko Haram hideouts.
Jonathan issued a decree allowing soldiers to arrest people at will and take over buildings suspected of housing fighters.
Analysts have said that at the heart of the surge in violence is dire poverty and political manoeuvring - not religion.
They believe attacks were committed mainly by frustrated, unemployed youths and orchestrated by religious leaders and politicians who manipulate them to retain power.
Despite the deployment of more soldiers and police to northern Nigeria, the nation's weak central government has been unable to stop the killings.
From 2010 on, attacks that have killed more than 1,500 people according to a count by the Associated Press.