Specifically, a letter published by Wired revealed that he asked Brennan about the rules governing the use of drone aircraft to assassinate American citizens overseas — a topic the administration has been rather secretive about.

“Senior intelligence officials have said publicly that they have the authority to knowingly use lethal force against Americans in the course of counterterrorism operations, and have indicated that there are secret legal opinions issued by the Justice Department’s Office of Legal Counsel that explain the basis for this authority,” Wyden wrote. “I have asked repeatedly to see these opinions, and I have been provided with some relevant information on the topic, but I have yet to see the opinions themselves.”