Anwar al-Awlaki was described as "chief of external operations" for al-Qaeda in the Arabian Peninsula.
Awlaki was a western-educated American citizen who spoke fluent English who rejected his adopted country for the militant ideology of al-Qaeda.
"Don't consult with anybody in the killing of Americans. Fighting the devil doesn't require consultation or prayers seeking divine guidance," he has said. "They are the party of the devils."
Awlaki studied civil engineering in the United States but worked as a Muslim preacher and lecturer of religion in the late 1990s.
He travelled to Yemen in 2004, where he established contact with Islamist groups fighting the government in Sanaa.
Awlaki is thought to have helped oversee a plot in October 2010 to detonate explosives aboard a US-bound cargo plane.
He is also said to have had contact with Major Nidal Hassan, the man behind the Ford Hood army base shootings in 2009 and to have played a role in the attempted "Christmas Day" attack later that year.
"Yes, Nidal Hassan was one of my students and I am proud of that. I am proud of Nidal Hasan and this was a heroic and wonderful act," Awlaki has said.
Al Jazeera's Khadijah Magardie reports.