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U.S. authorities said Thursday that they have captured a top Al Qaeda leader who allegedly masterminded the 2000 attack on the USS Cole and is a suspect in the bombing of two U.S. Embassies in East Africa.

Abd al-Rahim al-Nashiri was identified as one of a dozen key terrorist leaders about whom the FBI had sought information. Federal law-enforcement officials called his recent arrest a major setback to Osama bin Laden’s plans to strike again.

“This is a serious blow to Al Qaeda and a significant success in the fight against terrorism to catch a guy like this,” said one high-level official.

Word of the capture surfaced last week. Congressional Democrats were questioning the wisdom of starting a war with Iraq when many senior Al Qaeda leaders were still on the loose and a fresh audiotape thought to have been made by bin Laden threatened more strikes against the U.S. and its allies.

Administration officials responded by insisting they had made progress in combating Al Qaeda. Unidentified officials quoted in news reports said a senior Al Qaeda member had been seized but didn’t name him.

On Sunday, Homeland Security Director Tom Ridge confirmed that a top member of Al Qaeda had been apprehended, but again declined to give details. Ridge said the detainee was providing U.S. interrogators with insight into Al Qaeda’s operations.

On Thursday, U.S. officials gave few details of the capture or of al-Nashiri’s status. They said he was arrested overseas this month and is in U.S. custody. Officials would not say where he was being held, or why they were confirming his identity now.

A Justice Department official, while confirming the capture, said al-Nashiri, who is close to bin Laden, is not in their custody but that the matter was being handled by other federal agencies abroad.

“He’s one of the most top-wanted guys. He’s on the list,” the Justice official said. “We don’t have him in our custody, but he could be in military custody or in the custody of some foreign government with our guys getting access to him.”

Al-Nashiri is considered the most senior Al Qaeda operative arrested since the capture of Abu Zubaydah, bin Laden’s operations chief, in Pakistan last March. Top U.S. officials on Thursday described him as the terrorist network’s chief operative in the Persian Gulf.

U.S. intelligence considers the Saudi-born al-Nashiri to be the brains behind the attack on the USS Cole off Yemen, when two suicide bombers detonated an explosive on the ship’s hull, killing 17 U.S. sailors. Al-Nashiri also is believed to have had a hand in the U.S. Embassy bombings in Kenya and Tanzania, which killed 224 people.

Authorities said al-Nashiri also planned the failed plot to bomb another destroyer docked in Yemen, the USS The Sullivans, nine months before the Cole attack. Al-Nashiri, reportedly an expert in explosives and weapons training, has been involved in plotting other attacks against American military targets in the Persian Gulf region and “possibly elsewhere,” sources added.

A law-enforcement watch list distributed to police agencies across the U.S. in the wake of the Sept. 11 attacks identifies him as a “known Al Qaeda operative” who also goes by the name of Abdurahim Al Nashiri.

The watch list said al-Nashiri attended a training camp operated by Al Qaeda in Afghanistan in 1997, after meeting Al Qaeda operatives four years earlier in Bosnia-Herzegovina.

In the plot against the USS The Sullivans, the watch list says, the attack failed when the small boat al-Nashiri financed sank because the suicide bombers overloaded it with explosives.

If reports that al-Nashiri is cooperating with his interrogators are true, he would join bin Laden deputy Zubaydah in providing information. Officials have said Zubaydah helped lead them to Jose Padilla, a U.S. citizen picked up in Chicago for allegedly planning to scope out targets for a radioactive “dirty bomb.”

“This guy has a track record,” one defense official at the Pentagon said of al-Nashiri. “What he knows would be all current terrorist plans in that part of the world. He would know how the money moves in, how the money moves out. He’s going to know the operators who are going to carry out the attacks. He’s going to know the location of safe houses … and he’s going to know how explosives get in and where they come from.”