Donald Trump's former campaign chairman is suing special counsel Robert Mueller and the Justice Department.
Paul Manafort alleges in the lawsuit that Mr Mueller exceeded his authority by investigating him for conduct that was not related to Russian interference in the 2016 presidential election.
Mr Manafort was indicted in October and has pleaded not guilty. The suit was filed on Wednesday in federal court in Washington.
The move comes amid Republican allegations of partisan bias among members of Mr Mueller's team, which for months has been investigating whether the Trump campaign coordinated with Russia to influence the outcome of the US election.
Mr Manafort was indicted in October on charges related to his lobbying work on behalf of a Russia-friendly Ukrainian political party. He has pleaded not guilty.
He alleges in his lawsuit that the investigation into "decade-old business dealings" is "completely unmoored" from the mandate Mr Mueller was given when he was appointed in May by Deputy Attorney General Rod Rosenstein.
"Those alleged dealings had no connection whatsoever to the 2016 presidential election or even to Donald Trump. Nor were they uncovered in the course of the Special Counsel's probe into President Trump's campaign," the suit alleges.
The lawsuit also argues that Mr Rosenstein's order appointing Mr Mueller was overly broad and arbitrary.