President Donald Trump lashed out at the appointment of a special counsel made by his own justice department to oversee the federal investigation into Russian interference in the 2016 election and possible collusion with his campaign.

He tweeted that this is the single greatest witch hunt of a politician in American history

The Justice Department appointed former FBI director Robert Mueller as special counsel to oversee the investigation, which has focused on contacts between trump’s campaign associates and Russian officials.

The move aimed to quell the wave of criticism facing the Trump administration since the President fired Comey last week as the investigation intensified.