Buried in this Washington Post story about Nobel Peace laureate Obama's Newest War, in Yemen:
After the Sept. 11 attacks, Bush gave the CIA, and later the military, authority to kill U.S. citizens abroad if strong evidence existed that an American was involved in organizing or carrying out terrorist actions against the United States or U.S. interests, military and intelligence officials said. The evidence has to meet a certain, defined threshold. The person, for instance, has to pose "a continuing and imminent threat to U.S. persons and interests," said one former intelligence official.
The Obama administration has adopted the same stance. If a U.S. citizen joins al-Qaeda, "it doesn't really change anything from the standpoint of whether we can target them," a senior administration official said. "They are then part of the enemy."