Former FBI Deputy Director Andrew McCabe drafted a memo on circumstances leading up to the firing of his onetime boss, James Comey, a person familiar with the situation said Wednesday night.

The person spoke on condition of anonymity to The Associated Press to discuss a secret document that has been turned over to special counsel Robert Mueller. His team is investigating whether the Trump campaign coordinated with Russia during the 2016 presidential election and whether the president sought to obstruct that inquiry through actions including the firing of Comey last May.

The memo concerns a conversation that McCabe had with Deputy Attorney General Rod Rosenstein about Rosenstein's preparations for Comey's firing. Rosenstein played an important role in that episode, having authored a memo faulting Comey for his handling of the Hillary Clinton email investigation that the White House held up as justification for President Donald Trump's decision to fire the FBI director.