WASHINGTON (Reuters) - The government has authorized the killing of American citizens as part of its controversial drone campaign against al Qaeda even without intelligence that such Americans are actively plotting to attack a U.S. target, according to a Justice Department memo. The unclassified memo, first obtained by NBC News, argues that drone strikes are justified under American law if a targeted U.S. citizen had "recently" been involved in "activities" posing a possible threat and provided that there is no evidence suggesting the individual "renounced or abandoned" such activities. The document was disclosed as a bipartisan group of U.S. senators...