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SUNDAY ALAMBA / THE ASSOCIATED PRESS ARCHIVES A Nigerian soldier secures the scene of the car-bombed United Nations office in Nigeria, Aug. 27.

ABUJA, Nigeria -- Nigeria detained and released several radical Muslims suspected of being terrorists in 2007 -- including a man who officials now say helped organize last week's deadly car bombing at the United Nations headquarters in the nation's capital, a high-ranking official told The Associated Press.

The men arrested four years ago had allegedly been caught with explosives. Their rapid release from detention was apparently aimed at placating Muslim groups, but it has now come back to haunt security officials who fear a growing wave of al-Qaida-linked terror attacks in Nigeria, a main supplier of oil to the United States.

Some of those arrested in October 2007 were even plotting to carry out attacks in the United States and to attack American targets here, in Africa's most populous country, said the official, who claimed direct knowledge of the arrests. He spoke on condition of anonymity due to the political sensitivity of the case and because he is not authorized to discuss the matter with journalists.

Nigeria remains very sensitive to any suggestion it is a haven for terrorists, and the information released at the time of the arrests was fairly vague. It was not immediately clear if Nigeria shared information about the purported anti-U.S. plots with U.S. officials. The U.S. Embassy had no immediate comment Thursday.

However, in a report on global terror threats, the State Department said diplomats issued a warning to U.S. citizens in 2007 about possible attacks on U.S. and western interests in Nigeria. It also noted that Nigerian authorities said they arrested at least 10 suspected terrorists in northern Nigeria late that year with alleged ties to al-Qaida in the Islamic Maghreb.

A former U.S. ambassador to Nigeria, John Campbell, who left the country several months before the 2007 arrests, said Pakistanis would have stood out in northern Nigeria. Campbell, who is now a fellow at the Council on Foreign Relations, said he had no information about arrests of any Pakistanis.

Top security officials in the administration of then-president Umaru Yar'Adua, a Muslim, released the rounded-up men shortly after their arrests, with some facing a few hasty sham trials, the Nigerian official said.

One of those men was Babagana Ismail Kwaljima, also known as Abu Summaya, who was arrested again days before the Aug. 26 bombing at the UN compound in Abuja that killed at least 23 people, the Nigerian official said. Kwaljima is accused of helping mastermind the UN bombing. A second man was also arrested and police are looking for a third with "al-Qaida links" who recently travelled in Somalia, where an al-Qaida-linked group called al-Shabab is battling the beleaguered UN-backed government.

Kwaljima is being held at a military base in Nigeria, according to Nigeria's secret police. The agency previously arrested him in October 2007 in the northern city of Kano during a roundup of suspected members of al-Qaida in the Islamic Maghreb operating in the country, the official who spoke to AP said. AQIM, as the group is known, generally operates in Saharan nations north of Nigeria.

Secret police spokeswoman Marilyn Ogar declined to comment on Thursday.

Suspected Pakistani members of al-Qaida were arrested in October 2007 along with members of AQIM, the official said. He did not provide numbers of people arrested. News reports that emerged in November 2007 about arrests in the area also did not specify numbers, but identified the men as Nigerians. No foreigners were mentioned.

The official told AP that AQIM was planning to carry out terror attacks against targets in the United States and the Pakistanis were plotting terror attacks against U.S. citizens working in Nigeria, which is divided into a mainly Christian south and Muslim north.

"They were caught with explosive devices and other ammunitions. Some of them were also caught with large amount of cash," the Nigerian official said.

-- The Associated Press