The U.S. Justice Department gave the approval to the CIA to kill Anwar al-Awlaki, the charismatic U.S.-born cleric who orchestrated Al Qaeda recruitment from Yemen, by justifying his killing as an act of self-defense, a U.S. official told Fox News.

The official said Sunday that a secret memo between the DOJ and CIA placed al-Awlaki on a kill or capture list, which laid out the case that al-Awlaki was not entitled to the same protections as a U.S. citizen, but rather a combatant targeting U.S. citizens.

Al-Awlaki, who was killed Friday in a CIA-led drone strike, has been described as an operational planner with Al Qaeda in the Arabian Peninsula. But President Obama said he was the leader of "external operations" for the Al Qaeda affiliate.

As such, Obama said, the U.S.-born radical cleric "took the lead" in planning attacks on Americans and called on others to take part in the "murderous agenda."

Al-Awlaki, born in New Mexico to Yemeni parents, was believed to be key in turning Al Qaeda's affiliate in Yemen into what American officials have called the most significant and immediate threat to the United States. The branch, led by a Yemeni militant named Nasser al-Wahishi, plotted several failed attacks on U.S. soil -- the botched Christmas 2009 attempt to blow up an American airliner heading to Detroit and a foiled 2010 attempt to send explosives to Chicago.

The killing, however, drew some rebuke from civil liberties groups and a few politicians who say the U.S. should not perform these types of strikes.

Rep. Dennis Kucinich, D-Ohio, joined Rep. Paul, R-Texas, in condemning the killing of al-Awlaki, saying the Obama administration had "crossed a dangerous divide and set a dangerous precedent for how the United States handle terrorism cases."

"Mr. al-Awlaki's allegedly violent rejection of America was not acceptable in any way," Kucinich said. "Neither is it acceptable to trample the Constitution through extrajudicial killings."