The Justice Department secretly concluded in July 2010 that the U.S. military or the CIA could legally use a drone to kill Anwar al-Awlaki, accused of being an al Qaeda leader, even though he was an American citizen.
Fourteen months later, he was killed in a drone strike in Yemen.
A declassified version of the memo was made public Monday by a federal court after the Obama administration declined to appeal an earlier ruling that required its disclosure. The New York Times and the ACLU sought its release under the Freedom of Information Act.
The memo prepared by the Justice Department sets out a series of arguments for why the assassination would be legal.
"We believe DoD's contemplated action against al- Aulaqi would comply with international law, including the laws of war applicable to this armed conflict and would fall within Congress' authorization to use 'necessary and appropriate force' against al-Qaida," the memo said.
Al-Awlaki, the memo concluded, was "engaged in continual planning and direction of attacks upon U.S. persons from one of the enemy's overseas basis of operations, the U.S. government does not know precisely when such attacks will occur, and a capture operation would be infeasible."
Obama administration officials, including Attorney General Eric Holder, had earlier given a public explanation of the broad outlines of the memo's legal arguments.
While a federal law makes it a crime to murder an American overseas, such a killing is not a crime "if done with the proper public authority," the memo argued. It said similar logic allows a policeman to kill a suspect when deadly force is justified. And a soldier is not guilty of murder when killing an enemy in time of war.
Al-Awlaki had assumed operational and leadership roles in al Qaeda in Yemen and continued "to plot attacks intended to kill Americans from his base of operations" there.
Under the authorization for the use of military force approved by Congress in 2001, the memo said, the U.S. had a legal basis for using lethal force, even against an American citizen who was part of the forces of an enemy organization, such as al Qaeda, it said.
The fact that the active battlefield was in Iraq and Afghanistan while al-Awlaki was in Yemen posed a further complication for the administration. But the memo said government lawyers could find no prohibition to targeting him.
"We have not come across any authority for the proposition that when one of the parties to an armed conflict plans and executes operations from a base in a new nation, an operation to engage the enemy in that location can never be part of the original armed conflict," the memo said.
The memo tackled a section of U.S. law that prohibits the murder of American citizens by other U.S. nationals in foreign lands and said that targeting al-Awlaki with a drone would be an exception.
"Nothing in the text or legislative history of section 1119 indicates that Congress intended to criminalize such an operation," the memo says.
ACLU deputy legal director Jameel Jaffer said the memo's release "represents an overdue but nonetheless crucial step towards transparency."
"The drone program has been responsible for the deaths of thousands of people, including countless innocent bystanders, but the American public knows scandalously little about who is being killed and why," he said in a statement.
"There are few questions more important than the question of when the government has the authority to kill its own citizens."