https://www.theguardian.com/uk-news/2017/apr/13/british-spies-first-to-spot-trump-team-links-russia?CMP=Share_iOSApp_Other
According to one account, GCHQ’s then head, Robert Hannigan, passed material in summer 2016 to the CIA chief, John Brennan. The matter was deemed so sensitive it was handled at “director level”. After an initially slow start, Brennan used GCHQ information and intelligence from other partners to launch a major inter-agency investigation.
In late August and September Brennan gave a series of classified briefings to the Gang of Eight, the top-ranking Democratic and Republican leaders in the House and Senate. He told them the agency had evidence the Kremlin might be trying to help Trump to win the presidency, the New York Times reported.
One person familiar with the matter said Brennan did not reveal sources but made reference to the fact that America’s intelligence allies had provided information. Trump subsequently learned of GCHQ’s role, the person said.
Edited by Silas at 22:27:47 on 04/13/17