Abuja, Nigeria - Investigators probed an Islamist sect on Monday over the suicide bombing of the UN headquarters in Nigeria that killed at least 21 people in one of the bloodiest attacks targeting the world body.
Fresh violence began on Sunday when gunmen stormed the home of a local government chairperson in the northeast and shot him dead. An attacker also threw a bomb onto the property of an ex-government minister, but there were no injuries.
The UN's deputy chief held talks with Nigerian President Goodluck Jonathan on Sunday and afterward said the "perpetrators must be brought to justice," while vowing that the attack would not deter the work of the world body.
Deputy UN Secretary General Asha-Rose Migiro was due to leave Nigeria on Monday, but United Nations security chief Gregory Starr would remain in the country for now as the probe continues, said UN spokesperson Martin Dawes.
No arrests yet
Police said an all-encompassing investigation was being carried out that would look at the Islamist sect known as Boko Haram, which has claimed responsibility, as well as a range of other factors and past attacks.
"It's a multi-dimensional investigation so that a complete stop can be put to these incidences," said police spokesman Yemi Ajayi. "Boko Haram is still part of what we are looking at."
No arrests had been made. He declined to reveal who was involved in the investigation, but the US embassy has confirmed that FBI agents were in Nigeria.
The sect has claimed responsibility for the attack, but analysts have cautioned that while at least one faction of the Islamist group may be involved, it was too early to draw firm conclusions.
Friday's attack saw the bomber make his way through two gates at the heavily guarded compound before slamming his car into the entrance of the building. In addition to the 21 killed, 73 have been confirmed injured.
Some 400 people from a variety of nationalities worked in the building.
Fear of bodies still in building
The vast majority of the dead were Nigerians, said Dawes. A 30-year-old Norwegian woman has also been confirmed among the dead.
A spokesperson at the national hospital said six of the wounded have been flown out to South Africa, but he could not provide further details. He also said some 200 Nigerians had donated blood.
Searches pressed ahead at the heavily damaged building.
"There is a fear that there could still be bodies in the building," Dawes told AFP. "A UN driver is still missing ... Search dogs have not found anything."
Boko Haram has been blamed for scores of shootings and bomb blasts, mainly in Nigeria's northeast, but it has not been known to target international institutions such as the UN.
It claimed a bomb attack targeting national police headquarters in Abuja in June that killed at least two people.
There have however been growing fears over whether members of the sect have formed links with groups outside Nigeria, including al-Qaeda's north African branch.
Political links
There has also been intense speculation over whether some of the violence blamed on the sect has been politically linked, and Nigerians speak privately about their suspicions that politics somehow played a role in the UN bombing.
Two incidents on Sunday were more typical of the attacks that have been blamed on Boko Haram, though authorities have not yet named suspects in those cases.
In one, gunmen stormed the home of a local government chairperson in Nigeria's northeast and shot him dead, according to a state official.
The attackers killed Lawan Yaraye, chairperson of Kukawa district in Borno state, where most of the attacks blamed on the sect have occurred. His home was in the city of Maiduguri, the epicentre of the violence.
The second incident saw a bomb thrown onto the property of Nigeria's ex-police minister, but it exploded without wounding anyone, a police source said.
The incident on Sunday night in the northern city of Bauchi targeted the home of Ibrahim Yakubu Lame, who was police affairs minister during an uprising by Boko Haram in 2009 put down by a brutal military assault.
Nigeria, Africa's most populous nation and the continent's largest oil producer, has a population of some 150 million roughly divided in half between a mainly Muslim north and predominately Christian south.