Nigerian authorities say a man with links to al-Qaida helped orchestrate last week's attack on the U.N. headquarters in Abuja that killed 23 people.
Nigeria's State Security Service said Wednesday that Mamman Nur, a member of the radical Islamist sect Boko Haram, masterminded the bombing while working "in concert" with two other suspects who have been arrested.
The statement said Nur had recently returned from Somalia, where al-Qaida is active assisting the insurgent group al-Shabab.
Nur remains at large. The arrested suspects were identified as Ismail Kwaljima and Babagan Mali, also said to be members of Boko Haram.
The State Security Service said they were arrested on August 21, five days before the bombing.
The service said it had received intelligence about a possible Boko Haram attack in Abuja three days before that, on August 18. It did not say why it was unable to prevent the attack.
Last Friday, a suicide bomber detonated a car bomb inside the U.N. Abuja compound, wounding more than 80 people in addition to the 23 deaths.
A man who identified himself as a Boko Haram spokesman told VOA that the group was responsible for the attack.
Boko Haram, whose name means "Western education is sinful," has been blamed for many bombings and shootings of authority figures in northeastern Nigeria, mainly in the state of Borno. The group has also claimed responsibility for attacks in Abuja, including a June bombing outside Nigeria's national police headquarters.
The group wants a strict form of Islamic law applied more widely across Nigeria.
Nigerian President Goodluck Jonathan has vowed to increase security and bring terrorism in the country under control.
Some information for this report was provided by AP and AFP.