ABUJA  (AFP) – The Federal Government Wednesday said  that an Al-Qaeda linked suspect who recently returned from Somalia masterminded last week’s attack on UN headquarters here, one of the bloodiest targeting the world body.

The statement by Nigeria’s secret police over Friday’s suicide bomb attack that killed at least 23 people came amid mounting concern over whether local Islamist sect Boko Haram has formed ties with outside extremist groups.

It also said that two other suspects, identified as key figures of the Boko Haram extremist sect, were arrested on August 21, days before the UN bombing.

Boko Haram  claimed responsibility for the bombing that also wounded dozens of others.

“Investigation has revealed that one Mamman Nur, a notorious Boko Haram element with Al-Qaeda links who returned recently from Somalia, working in concert with the two (arrested) suspects masterminded the attack on the United Nations building in Abuja,” the police statement said.

The suspect has been declared wanted following the attack on the building where some 400 UN staff with a variety of nationalities worked, it noted.

Nur’s name has previously circulated as a top figure within Boko Haram and he was considered by some to be the sect’s third-in-command during its 2009 uprising in northern Nigeria, put down by a brutal military assault.

He was believed to have fled to Chad to escape arrest by Nigerian security agents before moving on to Somalia, according to sources claiming to have been sect members as well as others familiar with Boko Haram.

Nur is thought to have returned to Nigeria several weeks ago at the beginning of the Muslim holy month of Ramadan.

The statement also described the two other suspects arrested on August 21 as “notorious leaders of the Boko Haram extremists sect.”

They were identified as Babagana Ismail Kwaljima, aka Abu Summaya, and Babagana Mali, aka Bulama.

Their arrests came after authorities received intelligence on August 18 of plans for attacks in the Nigerian capital Abuja, it said.

“On 18th August, 2011, precise intelligence was obtained by this service that some Boko Haram elements were on a mission to attack unspecified targets in Abuja …,” according to the statement.

“Following their arrests, security was further beefed up in Abuja and its environs,” the statement said. “Meanwhile the suspects have made valuable statements and are being held at a military facility.”

There has been growing concern over whether Boko Haram has formed links with extremist groups outside Nigeria, including Al-Qaeda’s north African branch and Somalia’s Shebab fighters.

Alleged sect members have claimed that they have received training in foreign countries, and analysts point out that their attacks have grown increasingly sophisticated.

Boko Haram has been blamed for scores of shootings and bomb blasts, mainly in Nigeria’s northeast, but it had 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.

In Friday’s attack, the bomber made his way past two gates before ramming his car into the entrance of the building.

The police statement gave what it said were details on the car used in the attack, saying it was a Honda registered in northern Kano state that had been purchased in 2002.

The UN’s deputy chief Asha-Rose Migiro 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 United Nations.

FBI agents from the United States have been asked to assist in the investigation.

Nigeria is Africa’s most populous nation with some 150 million people, roughly divided in half between a mainly Muslim north and predominately Christian south.