At a tense hearing in a federal court in Virginia on Friday, U.S. District Judge T.S. Ellis III sharply questioned whether Mueller exceeded his authority in filing tax and bank fraud charges against Trump's former campaign manager, Paul Manafort.

Ellis said the indictment appeared to be a way for Mueller to leverage Manafort into providing information about Trump.

"The vernacular is to sing," he said.

"You don't really care about Mr. Manafort," the judge said. "You really care about what information Mr. Manafort can give you to lead to Mr. Trump" and his eventual prosecution or impeachment.