White Paper

[T]he United States would be able to use lethal force against a US citizen, who is located outside the United States and is an operational leader continually planning attacks against US persons and interests, in at least the following circumstances: (1) where an informed, high-level official of the US government has determined that the targeted individual poses an imminent threat of violent attack against the United States; (2) where a capture operation would be infeasible -- and where those conducting the operation continue to monitor whether capture becomes feasible; and (3) where such an operation would be conducted consistent with applicable law of war principles.  In these circumstances, the "realities" of the conflict and the weight of the government's interest in protecting its citizens from an imminent attack are such that the Constitution would not require the government to provide further process to such a US citizen before using lethal force. ...

Finally, the Department notes that under the circumstances described in this paper, there exists no appropriate judicial forum to evaluate these constitutional considerations.

-- Department of Justice White Paper, Lawfulness of a Lethal Operation Directed Against a US Citizen who is a Senior Operational Leader of Al-Qa'ida or An Associated Force, excerpt, NBC News, 4 February 2013