Boco Haram told the BBC it carried out last week's attack that left dozens of people dead and many more injured at the U.N. offices in Abuja.
A car bomb crashed through security barriers into the reception area, witnesses told the BBC. U.S. Army Gen. Carter Ham, the top U.S. military authority working in Africa, had told the BBC earlier this month there were growing ties between Boko Haram and al-Qaida in the Islamic Maghreb, the North African branch of al-Qaida.
A review of internal intelligence reports from Nigeria by The Wall Street Journal found Boko Haram members were trained by al-Qaida affiliates in Afghanistan and Algeria.
The Nigerian report suggested Boko Haram groups began training with al-Qaida as early as 2002. A 2007 trip to Afghanistan was for training in the use of improvised explosive devices, the report adds. AQIM, which took responsibility for a suicide bombing of an Algerian military academy last week, also has a strong relationship with Boko Haram, the Journal notes.
A U.S. official who spoke with the newspaper on condition of anonymity said the Nigerian group has solidified its relationship with AQIM within the last year.
"What we're seeing now is probably the result of the additional radicalization of their viewpoints and the training," the official said.
Nigeria authorities announced Monday they've made several arrests in connection with last week's bombings but have provided few details. The United Nations said it was reviewing security details across the board in light of the attack.
Notable deaths of 2014 [PHOTOS]