Assistant Attorney General Sally Yates requested a private appointment with Trump and Pence. She then told them that Trump's National Security Advisor, Mike Flynn, was subject to blackmail by the Russians for his undisclosed activities on behalf of the Russians.

For 18 days, Trump and Pence sat on this information and allowed Flynn to sit on in meetings of critical national importance and all reports from our intelligence agencies.

Trump fired Sally Yates, bearer of this bad news about Russian interference, as he would later also fire Comey.

Despite this attempt at a cover up, the newspapers got hold of the story and Trump was forced to fire Flynn.

Mueller is reportedly zeroing in on the 18 days that passed between White House officials learning of Michael Flynn’s susceptibility to blackmail by Russians and his firing.

Focus on Flynn, Trump timeline suggests obstruction is on Mueller'''s mind - NBC News