David Jackson

USA TODAY

WASHINGTON -- The United States can use lethal force against a U.S. citizen overseas if that person is part of an enemy organization seeking to attack the U.S., according to a once-secret memo released Monday.

The 2011 killing of one such American -- al-Qaeda leader Anwar al-Awlaki -- was justified because it was carried out under applicable laws of war, the memo said.

The Obama administration document became public under a court order in a Freedom of Information Act lawsuit filed by the American Civil Liberties Union and The New York Times.

"The release of this memo represents an overdue but nonetheless crucial step towards transparency," said ACLU Deputy Legal Director Jameel Jaffer. "There are few questions more important than the question of when the government has the authority to kill its own citizens."

In the memo dated July 16, 2010, then-Acting Assistant Attorney General David Barron wrote that "the U.S. citizen in question has gone overseas and become part of the forces of an enemy with which the United States is engaged in an armed conflict."

In addition, the target of the drone strike "is engaged in continual planning and direction of attacks upon U.S. persons from one of the enemy's overseas bases of operations," Barron wrote.

The drone strike that killed al-Awlaki came in September of 2011. Two other Americans also died in the attack, including al-Awlaki's 16-year-old son; the government said they were not targeted.

The 41-page document included redactions that the government said involved national security secrets.

Jaffer said the ACLU and others would continue pursuing documents about the uses of drones.

"The drone program has been responsible for the deaths of thousands of people, including countless innocent bystanders," he said. "But the American public knows scandalously little about who is being killed and why."