The fugitive, known as Abu Anas al-Liby, possesses decades of intelligence into Al Qaeda, extending from the group's early days behind Osama bin Laden in Sudan to its more scattered fragments today.
Abu Anas, 49, who was born Nazih Abdul-Hamed al-Ruqai, has been described as a Qaeda computer expert who also helped to conduct surveillance of the American Embassy in Nairobi, Kenya, one of the two that were bombed. In the investigations after the attacks, the authorities also recovered a Qaeda terrorism manual in his residence in Manchester, England.
The manual is a detailed treatise about how to carry out successful terrorist attacks. It focuses on forged documents, safe houses, surveillance, assassinations, codes and interrogation techniques. It also cites "blasting and destroying the embassies and attacking vital economic centers." It also recommends the use of explosives in attacks because they "strike the enemy with sheer terror and fright."
It is not known if Abu Anas wrote the manual, but federal prosecutors introduced it as evidence against defendants in the 2001 trial of four operatives convicted in the embassy bombings conspiracy, and that of Ahmed Khalfan Ghailani, a first former detainee at Guantánamo Bay, Cuba, to be tried in the federal system. It was also used in a 2006 trial in Virginia over whether to impose the death penalty on Zacarias Moussaoui, the so-called 20th hijacker in the Sept. 11 plot ( he received a life sentence).
The Defense Department, in a statement on Sunday, said that Abu Anas was "currently lawfully detained under the law of war in a secure location outside of Libya."
The statement also suggested that the authorities were seeking to take an approach similar to what they had done in other cases of international terrorism in recent years - interrogating Abu Anas for intelligence purposes and only then advising him of his rights to a lawyer and a speedy court appearance.
Abu Anas is expected to face trial in the United States District Court in Manhattan, although it was not immediately clear when he would be brought to New York. The indictment first charged Bin Laden in 1998, and has since expanded to include Abu Anas and two dozen other defendants.
With Abu Anas's capture, only three key Qaeda operatives charged in those indictments are believed to be alive and still at large, most prominently Ayman al Zawahiri, the deputy to Bin Laden who succeeded him after he was killed in a 2011 American operation.
One of Bin Laden's former close aides, a Sudanese man who defected from the group in the mid-1990s and became a cooperating witness for the American government, testified that Abu Anas was a computer engineer. "He run our computers," the witness, Jamal Ahmed al-Fadl, testified in the 2001 trial.
Abu Anas was also part of a small team of Qaeda operatives that in the early 1990s traveled to Nairobi and carried out surveillance of the American Embassy and other potential bomb targets, according to the federal indictment and other prosecution evidence introduced at Qaeda trials.
The team met there with Bin Laden's military commander, Muhammad Atef, and an operative, Khaled al-Fawwaz, who led the Nairobi cell of Al Qaeda at the time, evidence and testimony has shown.
The photographs, diagrams and surveillance report from the Nairobi mission were ultimately reviewed by Bin Laden in Khartoum, the government has said. "Bin Laden looked at the picture of the American Embassy and pointed to where a truck could go as a suicide bomber," another member of the surveillance team, Ali A. Mohamed, said in federal court when he pleaded guilty to conspiracy in 2000.
Eric Schmitt contributed reporting from Washington.