Boko Haram, a militant Islamist group blamed for scores of assassinations and bombing attacks in Nigeria, has claimed responsibility for Friday's bombing that killed at least 18 people at the United Nations office in the Nigerian capital.
In an apparent suicide bombing, a car smashed through security barriers and exploded at the entrance of the UN headquarters in Abuja, destroying most of the ground floor of a building where 400 UN employees work.
The attack is a dramatic escalation in Boko Haram's terrorism campaign in Nigeria. Reports say the UN had been warned last month that it could be targeted for attack by Boko Haram. The group launched a similar attack in the same city in June, killing several people at the Nigerian police headquarters.
The group has committed similar bombings in northern Nigeria, including car bombings and other attacks that have killed or injured hundreds of people. According to U.S. military officials, Boko Haram has been developing an alliance with other terrorist groups, including groups in Somalia and North Africa that are linked to al-Qaeda, which has often attacked UN targets.
The group has claimed that its members have received training in Somalia, where a Qaeda-linked group is fighting against a UN-backed peacekeeping force.
The group's most commonly known name, Boko Haram, means literally "non-Islamic education is a sin." It seeks to destroy any form of secular government and impose Islamic law on Muslim regions of northern Nigeria. More recently it has expanded into a larger insurgency against the Nigerian government.
The group was created in 2002 by a Muslim cleric, Mohammed Yusuf. It was peaceful at first, but in 2009 it launched an armed uprising in an attempt to establish an Islamic state in northern Nigeria. Its rebellion was crushed by the Nigerian military, leaving more than 800 people dead. Most of the dead were Boko Haram members, including Mr. Yusuf himself, who was killed in police custody.
Since then, the group has gone underground, and its command structure is mysterious. But it has shown an ability to organize across many regions of northern Nigeria and even in other places such as Abuja.
Analysts say the rise of Boko Haram is more than a religious phenomenon. It has capitalized on widespread anger at government corruption, poverty and unemployment. There are also unconfirmed reports that the group has developed links to politicians in the north.
The Nigerian government's crackdown on Boko Haram since 2009 has been excessively heavy-handed, breeding further anger and militancy, analysts say.