>>2194517
"That summer, GCHQ’s then head, Robert Hannigan, flew to the US to personally brief CIA chief John Brennan. The matter was deemed so important that it was handled at “director level”, face-to-face between the two agency chiefs."
"ames Clapper, director of national intelligence, later confirmed the “sensitive” stream of intelligence from Europe. After a slow start, Brennan used the GCHQ information and other tip-offs to launch a major inter-agency investigation. Meanwhile, the FBI was receiving disturbing warnings from Steele."