Donald Trump and Robert Mueller

The U.S. Department of Justice announced Wednesday that it appointed former FBI chief Robert Mueller as a special counsel to investigate Russia’s alleged meddling in the 2016 presidential election.

A DOJ press release states Mueller will oversee an ongoing investigation into the Russian government’s alleged efforts to influence last year’s elections, as well as related matters. International media are reporting Mueller will probe possible collusion between President Donald Trump’s campaign and Moscow.

Mueller’s appointment comes a week following Trump’s firing of FBI head James Comey. The Comey firing enraged Trump’s political opponents and prompted a media uproar.

Trump responded to Mueller’s appointment with a statement saying there was no collusion between his election campaign and a foreign entity and that a thorough investigation will confirm that. Then on Thursday morning, the president published a tweet saying, “This is the single greatest witch hunt of a politician in American history.”