NEW YORK — A federal appeals court on Monday released a previously secret memo that provided legal justification for using drones to kill Americans suspected of terrorism overseas.

The memo concluded that the killing of an al-Qaida leader who had been born in the United States had legal justification. It said the authority to use lethal force abroad may apply in appropriate circumstances to a U.S. citizen who is part of the forces of an enemy organization. It said the killing was justified as long as it was carried out in accord with applicable laws of war.

The 2nd U.S. Circuit Court of Appeals in Manhattan released the memo, portions of which are blacked out, after the American Civil Liberties Union and The New York Times filed a Freedom of Information Act lawsuit.

Jameel Jaffer, an ACLU lawyer who argued the case before the 2nd Circuit, said the memo’s release “represents an overdue but nonetheless crucial step towards transparency. There are few questions more important than the question of when the government has the authority to kill its own citizens.”

The memo pertained specifically to the September 2011 drone-strike killing in Yemen of Anwar Al-Awlaki, an al-Qaida leader who had been born in the United States.

Some legal scholars and human rights activists complained that it was illegal for the U.S. to kill American citizens away from the battlefield without a trial.

Lawyers for the Times and ACLU had said that the government’s continued delays regarding the document were cheating the public of a fully informed and fair debate over the highly classified “targeted-killing” program.