Mueller report: Criminal probe into Russia inquiry begins
Washington DC: The US justice department has launched a criminal investigation into the origins of the Mueller inquiry, US media report.
An administrative review into the special counsel's investigation of 2016 election interference began in May.
But the switch to a criminal probe means investigators can now issue subpoenas for testimony and documents.
President Trump has long alleged Robert Mueller's probe of reports of collusion with Russia was a "witch hunt". The investigation into Russian interference in the 2016 election did not establish any criminal conspiracy between Moscow and Donald Trump's campaign. But it did not clear the president of obstructing justice.
Reports on the criminal probe first appeared in the New York Times. It is unclear what potential crime is under investigation, the newspaper said.
The administrative review of the Mueller investigation began in May. It is being overseen by the US Attorney-General William Barr and is run by US federal prosecutor John Durham.
Durham was tasked with determining whether the collection of intelligence on the Trump campaign in 2016 was lawful.
He is widely respected and known for investigating links between FBI agents and organised crime, and investigating the destruction of CIA interrogation videos.
Last April, Barr told members of Congress that he believed "spying did occur" on the Trump campaign in 2016, adding: "The question is whether it was adequately predicated. And I'm not suggesting that it wasn't adequately predicated. But I need to explore that."
Critics accused Barr of launching an administrative review more in the interests of the president than the interests of justice.