Nigerian authorities said yesterday that an Al-Qaeda linked suspect who recently returned from Somalia masterminded last week’s attack on UN headquarters in Abuja, 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 has 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.

Mr 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.