President Trump on Tuesday called George Papadopoulos – his campaign foreign policy adviser who pleaded guilty to lying about his Russian contacts to FBI agents – a “young, low level volunteer” and a “liar.”

As part of Robert Mueller’s investigation into Russia’s interference in the 2016 presidential election, Papadopoulos pleaded guilty to making false statements and “material omissions” to the FBI about several communications he had with allies of the Russian government, including a professor who offered him “dirt” on Trump’s election opponent, Hillary Clinton.

Additional charges were filed Monday against two major players in Trump’s campaign: former campaign chairman Paul Manafort and his business associate Rick Gates, who also worked on Trump’s campaign.

The series of charges unsealed early Monday morning is a significant step in the federal investigation of Russian attempts to influence the 2016 presidential election, which Trump has previously dismissed as a “hoax” and a “witch hunt.”

Yet the president pointed to Tony Podesta, a Democratic lobbyist and brother to former Clinton campaign chairman John Podesta, who has become a subject of Mueller’s investigation as “the biggest story yesterday.”