The Sept. 11 attacks triggered a revolution in U.S. spycraft as the intelligence services shattered a longstanding taboo by launching an expansive program of targeted killings by remote control. The greatest shift both in tactics and mindset has been the embrace of the pilotless, hunter-killer aircraft known as drones. The CIA, which doesn't formally acknowledge the covert program, has killed about 2,000 militants with drones, U.S. officials say, most in the past two years as President Barack Obama's national security team aggressively expanded the program. In 2010, the number of drone strikes more than doubled, to 114, and this year, drone campaigns are expanding. The CIA now plans flights in Yemen, and the military is using drones to kill militants in Somalia. Legal challenges to the drone program have secured little traction. The main debate inside the government has been over how to execute the campaign without irreversibly damaging Pakistani cooperation. American citizens can be targets, too. Under the legal authority for the drone program, the CIA must consult the National Security Council before capturing an American posing an imminent threat, but no additional consultation is required to kill an American, a former senior intelligence official said. The reason there hasn't been more of an outcry about it is, it's the Obama administration defending this authority," said the American Civil Liberties Union's Jameel Jaffer.