Officials told The Washington Post a Justice Department memo authorized the attack. They said the action was discussed beforehand by senior lawyers in the executive branch who agreed his status as a citizen should not protect him.
"What constitutes due process in this case is a due process in war," said one of the officials, who spoke on the condition of anonymity to discuss closely held deliberations within the administration.
Samir Khan, a young naturalized U.S. citizen who edited Inspire, a magazine aimed at young Muslim men, was traveling with Awlaki and was killed with him.
Intelligence officers were not aware of his presence, but an official said he was considered a legitimate target as a belligerent.
In a statement on the killing Friday, President Barack Obama called Awlaki the head of "external operations" for al-Qaida in the Arabian Peninsula.