Federal Judge Emmet Sullivan suggested that former Trump administration national security adviser Michael Flynn could have been charged with treason during a fiery tirade at the three-star general’s sentencing hearing Tuesday.

Special counsel Robert S. Mueller III and his team of prosecutors did not charge Flynn with treason, and multiple legal experts have said it would be extraordinarily difficult for them to prove such a charge.

When pressed by Sullivan on whether Flynn’s actions rose “to the level of treasonous activity” or could be prosecuted under the Logan Act prohibiting unauthorized U.S. persons from negotiating with foreign countries, prosecutors declined to answer.

Flynn has pleaded guilty to lying to the FBI about conversations he had with the Russian ambassador to the U.S. during Trump’s transition to the White House and about his work lobbying for the Turkish government through a conduit while he was working on the Trump campaign and transition team.

“I’m not hiding my disgust, my disdain for your criminal offense,” Sullivan told Flynn in the courtroom in Washington, D.C., around noon on Tuesday. “Arguably, you sold your country out. … In the White House! In the West Wing!”