W By Robin Wright and Richard A. Serrano

ASHINGTON – U.S. authorities said Thursday that they have captured a top al-Qaida 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 been seeking 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-Qaida and a significant success in the fight against terrorism to catch a guy like this,” said one high-placed official.

Word of the capture first surfaced last week. At that time, Congressional Democrats were questioning the wisdom of starting a war with Iraq when many senior al-Qaida leaders were still on the loose and a fresh audiotape made by bin Laden threatened more strikes against the United States and its allies.

Responding to their critics, administration officials insisted that they had made progress in combating al-Qaida. Unnamed officials were quoted in press reports as saying a senior al-Qaida member had been captured, but they refused to name him. On Sunday, Homeland Security Director Tom Ridge confirmed that a top al-Qaida member had been apprehended, but again declined to give details. Ridge said the detainee was providing U.S. interrogators with insight into al-Qaida’s operations.

Thursday, U.S. officials provided few details of the capture or of Nashiri’s status. They said he was arrested earlier this month at an undisclosed location abroad and is now in U.S. custody, although officials refused to say where he was being held, or why they were confirming his identity now.

A Justice Department official, while confirming the capture, said that 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.”

Nashiri is considered the most senior al-Qaida 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 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 and killed 17 U.S. sailors. He also is believed to have had a hand in the U.S. embassy bombings in Kenya and Tanzania, which claimed 224 lives.

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

A law enforcement “watch list,” distributed to police agencies around the United States in the wake of the New York and Washington airplane bombings, identifies him as a “known al-Qaida operative,” who also goes by the name of Abdurahim Al Nashiri.

The watch list said Nashiri attended an al-Qaida training camp in Afghanistan in 1997, after meeting al-Qaida operatives four years earlier in Bosnia.

For the failed attack on the USS The Sullivans, he paid the suicide bombers to purchase a smaller boat they were going to use, the watch list says. It sank when the plotters overloaded it with explosives.

If reports that Nashiri is cooperating with his interrogators are true, he would join bin Laden deputy Zubaydah in providing information. Officials have said that 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.”

Nashiri “has a lengthy track record in terrorism and long-term ties to other senior al-Qaida leaders,” said a senior U.S. official who requested anonymity. “He is a ruthless operator.”

“This guy has a track record,” said one defense official at the Pentagon. “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 throughout the region, and he’s going to know how explosives get in and where they come from.

“He’s going to have a lot of knowledge. And who knows, maybe he knows where Osama bin Laden is.”