Nigerian police are investigating Boko Haram, the Islamist sect that claimed responsibility for Friday's suicide car bombing of the UN headquarters in Abuja, Nigeria's capital, that killed at least 21 people.
The attack on the UN building in Abuja was one of the deadliest targeting the world body, Agence France-Presse says.
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Boko Haram claimed responsibility for the bombing in a phone call to the BBC's Hausa language service. So far there have been no arrests made, and Nigerian investigators say that Boko Haram is only part of the investigation.
Deputy UN Secretary General Asha-Rose Migiro held talks with Nigerian President Goodluck Jonathan on Sunday and afterwards said the "perpetrators must be brought to justice," with the probe into the attack continuing, AFP says.
Some analysts say the secretive Islamist group may have split into different factions.
AFP says the US embassy in Abuja confirmed that FBI agents were in Nigeria.
Violent attacks have continued. On Sunday, gunmen stormed the home of a local government official in Nigeria's northeast and shot him dead, AFP says. In a separate attack on Sunday, a bomb was thrown onto the property of an ex-government minister, but no injuries were reported.
Boko Haram have claimed responsibility for attacks on police stations, banks and beer gardens that have killed or injured hundreds of people in recent months.
In the UN attack in Abuja, one wing of the building was completely destroyed, while the lower floors were badly damaged in what media reports described as “total carnage."
UN chief Ban Ki-moon called the attack "an assault on those who devote their lives to helping others," the BBC reports.