The reported targeting of Mr. Awlaki “certainly raises the question of what rights a citizen has and what steps must be taken before he’s put on the list,” said Mr. Tierney, Democrat of Massachusetts and chairman of a House subcommittee on national security.

Image The C.I.A. has placed the American-born cleric Anwar al-Awlaki on a list for killing. Credit... Muhammad ud-Deen/Associated Press

Counterterrorism officials, with the support of Democrats and Republicans in Congress, say the drone missile strikes have proved to be an extraordinarily successful weapon against militants in the tribal areas of Pakistan, the location of all the known C.I.A. strikes except one in Yemen in 2002. By their count, the missiles have killed more than 500 militants since 2008, and a few dozen nearby civilians.

In the fullest administration statement to date, Harold Koh, the State Department’s legal adviser, said in a March 24 speech the drone strikes against Al Qaeda and its allies were lawful as part of the military action authorized by Congress after the Sept. 11, 2001, attacks, as well as under the general principle of self-defense. By those rules, he said, such targeted killing was not assassination, which is banned by executive order.

But the disclosure last month by news organizations that Mr. Awlaki, 39, had been added to the C.I.A. kill list shifted the terms of the legal debate in several ways. He is located far from hostilities in Afghanistan and Pakistan, where the perpetrators of 9/11 are believed to be hiding.

He is alleged to be affiliated with a Yemeni branch of Al Qaeda. Intelligence analysts believe that only recently he began to help plot strikes, including the failed attempt to bomb an airliner on Dec. 25.

Most significantly, he is an American, born in New Mexico, arguably protected by the Fifth Amendment’s guarantee not to be “deprived of life, liberty, or property, without due process of law.” In a traditional war, anyone allied with the enemy, regardless of citizenship, is a legitimate target; German-Americans who fought with the Nazis in World War II were given no special treatment.

But Ms. Divoll, the former C.I.A. lawyer, said some judicial process should be required before the government kills an American away from a traditional battlefield. In addition, she offered a practical argument for a review outside the executive branch: avoiding mistakes.