LAGOS, Nigeria - Nigeria's secret police said Wednesday it arrested two suspected members of a radical Muslim sect that organized a car bombing at the United Nations' headquarters in the oil-rich nation that killed 23 people.

The agency also said it sought a third man who they said had "al-Qaida links who returned recently from Somalia."

The State Security Service issued a statement Wednesday saying the two suspects were members of the radical Muslim group known locally as Boko Haram. But the statement raised new questions about how much the secretive agency knew before Friday's bombing that left another 81 people wounded - as it acknowledged it had warning about a coming attack and that the men were arrested before the driver ever made it to the UN compound.

The agency said it received word Aug 18 about a possible car bomb attack in Abuja. On Aug 21, the secret police said it arrested Babagana Ismail Kwaljima and Babagan Mali, two men they said had ties to Boko Haram and were planning the attack.

On Friday, a suicide bomber rammed through two sets of gates to reach the UN building's glass reception hall. There, the bomber detonated explosives powerful enough to bring down parts of the concrete structure and blow out glass windows from other buildings in the quiet Abuja neighborhood filled with diplomatic posts.

The agency did not offer details if it shared any of the intelligence with diplomats in Abuja, Nigeria's capital. The security chief for the United Nations previously said it received no specific warning about a coming attack.

The secret police say the two men are being held at a military base.

"The suspects have made valuable statements," the agency said.

The agency said a third suspect, Mamman Nur, remained at large.

"Investigation has revealed that one Mamman Nur, a notorious Boko Haram element with al-Qaida links who returned recently from Somalia, (worked) in concert with the two suspects (in) masterminding the attack on the United Nations building in Abuja," the agency said in its statement.