Special counsel Robert Mueller has charged former Trump national security adviser Michael Flynn with “willfully and knowingly” making “false, fictitious and fraudulent statements” to the FBI regarding conversations with Russia’s ambassador.

He is due to appear in court later Friday morning, a court spokesperson said. The White House declined to immediately comment.

Flynn is the first Trump administration official and the fourth connected to the campaign to be charged as part of Mueller’s investigation into possible collusion between the Russian government and members of Trump’s team, as well as potential obstruction of justice and financial crimes.

Trump’s former campaign chairman Paul Manafort and his deputy Rick Gates were indicted last month; they pleaded not guilty. And Trump campaign foreign policy adviser George Papadopoulos pleaded guilty for making a false statement to the FBI over contacts with officials connected to the Russian government.

The charge against Flynn is the first in Mueller’s probe that has reached someone in the Trump White House and is the latest sign that the special counsel investigation is intensifying.