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.