The suspect, born Nazih Abd al Hamid al-Ruqhay and known by his nom de guerre Abu Anas el-Liby, has been high on the list of the United States government’s most-wanted fugitives since at least 2000, when a New York court indicted him for his part in planning the embassy attacks. The F.B.I. had offered a bounty of up to $5 million for information leading to his capture.

The officials, speaking on the condition of anonymity because of the sensitivity of attempts to locate Abu Anas, declined to make public further details on his capture.

The capture or killing of Abu Anas could be another grave blow to what remains of the original Qaeda organization after a 12-year-old American campaign to capture or kill its leadership. Its founder, Osama bin Laden, was killed in 2011 in a compound in Pakistan.

Abu Anas is not believed to have played any role in the attack on the United States diplomatic mission in Benghazi last year, senior officials briefed on that investigation say, but he may have sought to build networks connecting what remains of the Qaeda organization to like-minded militants in his native Libya.

Senior officials of the Libyan transitional government said they were unaware of the abduction of Abu Anas or any operation to capture him.

If Abu Anas turns out to have been captured by Western or United States forces, it is sure to put a new strain on the fragile authority of the transitional government. Many Libyans are fiercely protective of their national sovereignty and independence, and many are also suspicious that the interim prime minister, Ali Zeidan, a human rights lawyer in Geneva during his years in Libya’s exiled opposition, may lean too much toward the West.

Abu Anas, a 49-year-old born in Tripoli, is believed to have joined Bin Laden’s organization as early as the early 1990s, when it was based in Sudan. He later moved to Britain, where he had been granted political asylum. United States prosecutors in New York charged him in a 2000 indictment with helping to conduct “visual and photographic surveillance” of the United States Embassy in Nairobi in 1993 and again in 1995. In the indictment, prosecutors said Abu Anas had discussed with another senior Qaeda figure the idea of attacking an American target in retaliation for the United States peacekeeping operation in Somalia.

After the 1998 bombing, the British police raided his apartment and found an 18-chapter terrorist training manual in Arabic. Entitled “Military Studies in the Jihad Against the Tyrants,” it included advice on car bombing, torture, sabotage and disguise.

Since the overthrow of Col. Muammar el-Qaddafi in 2011, Tripoli has slid steadily into lawlessness, with no strong central government or police presence. It has become a haven for militants seeking to avoid detection elsewhere, and United States government officials, speaking on condition of anonymity to discuss confidential information, have acknowledged in recent months that Abu Anas and other internationally wanted terrorists had been seen moving freely around the capital.

His seizure was first reported Saturday in a short online posting on Twitter by a Libyan-born counterterrorism analyst based in London, Noman Benotman. His capture coincided with a fierce gunfight that killed 15 Libyan soldiers at a checkpoint in a neighborhood southeast of Tripoli, near the homeland of Abu Anas’s clan. A spokesman for the Libyan Army general staff, Col. Ali Sheikhi, said five cars full of armed men in masks pulled up at the army checkpoint at 6:15 a.m. and opened fire at point-blank range. Gunmen also fired from other positions further away in a coordinated attack, he said, and four soldiers were wounded but survived the attack. It was not clear if the assault was related to the capture of Abu Anas.

The United States has sharply reduced it diplomatic presence in Libya since the attack that killed former Ambassador J. Christopher Stevens in Benghazi on Sept 11, 2012. But it maintains a robust effort there trying to study and track suspected terrorists.