He characterized the practice as an "absolutely vital tool" in combating terrorism in a 2005 interview.

But critics of the administration's counterterrorism policy say increased use of armed drones against terrorism suspects, instead of interrogations, comes at the expense of important intelligence gains.

"When you send a drone to kill a terrorist, you not only vaporize the terrorist, you vaporize all the intelligence in his brain and so you might as well be setting file cabinets in the CIA on fire," Thiessen says. "The destruction of intelligence that has taken place under this administration's watch is unfathomable."