...members of Mr Trump's presidential campaign, as well as other Trump associates, "had repeated contacts with senior Russian intelligence officials in the year before the election".
Mr Flynn could have broken a law - known as
the Logan Act - by conducting US diplomacy as a private citizen, before he was appointed as national security adviser.
The retired army lieutenant-general initially denied having discussed sanctions with Ambassador Sergei Kislyak. Vice-President Mike Pence publicly denied the allegations on his behalf.
The White House admitted it had been warned about the contacts on 26 January but President Trump initially concluded Mr Flynn had not broken any law.