A federal appeals court on Monday released a once-secret memo that provides the Obama administration's legal justification for using drones to kill American terror suspects abroad.

The memo, released by the 2nd U.S. Circuit Court of Appeals in Manhattan as the result of a lawsuit, pertained to the targeted killing of American-born radical cleric Anwar al-Awlaki. The document lays out the legal justification for ordering the strike, without the involvement of a court.

"High-level government officials have concluded ... [al-Awlaki] is a leader of al Qaeda in the Arabian Peninsula (AQAP) whose activities in Yemen pose a 'continued and imminent threat' of violence to United States persons and interests," the memo said.

Portions of the 41-page, July 2010 memo are blacked out.

To back up its findings, the memo said: "facts presented to us indicate that [al-Awlaki] has been involved, through his operational and leadership roles in AQAP, in an abortive attack within the United States and continued to plot attacks intended to kill Americans."

This appears to be a reference to the failed 2009 attack by the so-called "underwear bomber," and the attempt to blow up cargo jets with printer bombs the following year. In both cases, the bombs were built by Ibrahim al-Asiri, and the memo provides no clues as to al-Awlaki's direct role in the plots.

The memo 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 memo was released after the American Civil Liberties Union and The New York Times filed a Freedom of Information Act lawsuit.

In a written statement, ACLU Deputy Legal Director Jameel Jaffer called the release 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," Jaffer said.

The memo was address to the "attorney general" and written by then-Justice Department official David Barron, who was recently confirmed to the U.S. Court of Appeals for the 1st Circuit.

The group continues to seek other documents regarding the lethal drone program, particularly pertaining to the deaths of Awlaki and two other Americans associated with him.

Al-Awlaki was born in New Mexico and was an imam at a Falls Church, Va., mosque more than a decade ago, before eventually returning to Yemen and allegedly linking up with Al Qaeda in the Arabian Peninsula.

He produced tapes urging attacks on Americans and was said to have had contact with Umar Farouk Abdulmutallab, the "underwear bomber."

Fox News' Catherine Herridge and The Associated Press contributed to this report.