“Awlaki’s name still pops up pretty often in cases of Western radicals, but given the amount of time since his death, it is unusual to see a case where the suspects actually met him,” said J. M. Berger, a fellow with the Brookings Institution’s project on American relations with the Islamic world who has studied Mr. Awlaki. “It reflects the long lead time on this plot. We may never know if this attack was formulated back then, or if the targets or particulars changed over time.”

American Origins

Mr. Awlaki was born in New Mexico in 1971 while his Yemeni father was a graduate student, went with his family to Yemen at the age of 7 and returned to the United States at 19 to study engineering at Colorado State University. He discovered a knack for preaching and spent eight years as a highly successful imam at mosques in Denver, San Diego and Washington, where he preached at the Capitol and was a luncheon speaker at the Pentagon.

He came under F.B.I. scrutiny briefly in 1999 for contacts with known militants, and again in 2002 when agents discovered that three of the Sept. 11, 2001, hijackers had worshiped in his mosques. The national Sept. 11 commission raised the possibility that Mr. Awlaki was part of a support network for the hijackers, but the F.B.I. concluded that he had no prior knowledge of the plot.

In 2002, Mr. Awlaki moved to London, where he became a popular speaker and flirted more openly with militancy. After moving to Yemen in 2004, he began to espouse violent jihad against the United States and other countries he labeled enemies of Islam.

By 2009, when Mr. Awlaki was linked to Maj. Nidal Hasan, the Army psychiatrist who killed 13 people in a shooting at Fort Hood, Tex., the F.B.I., as well as the authorities in Canada and Britain, found that the cleric’s calls for violence were turning up on the laptops of nearly everyone they charged with plotting jihadist attacks. His website and Facebook page had attracted a large following across the English-speaking world, and scores of foreigners traveled to Yemen to meet him.

“Awlaki was a huge magnet,” said Morten Storm, a Danish man who visited the cleric in Yemen, first as a convinced militant, and later, after growing disillusioned with Islam, as an agent of Danish, British and American intelligence agencies. Mr. Storm said the leader of A.Q.A.P., Nasir al-Wuhayshi, a former secretary to Osama bin Laden who now is the second-ranking figure in the global Qaeda network, remained a revered figure among jihadists.