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.