The US justice department's administrative review of the Mueller inquiry has been upgraded to a criminal investigation, US media report.
The Mueller report into the 2016 US elections did not establish any criminal conspiracy between the Trump campaign and Moscow.
However, it did not clear the president of collusion.
President Trump has repeatedly attacked the Russia investigation since it began and accused those involved of treason.
The change from a review to a criminal inquiry means that investigators can now issue subpoenas for witness testimony and documents.
The New York Times says it is not clear what potential crime is being investigated.
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 attorney John Durham.
Mr Durham was tasked with determining whether the collection of intelligence on the Trump campaign was lawful.
Last April, Mr 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."
The 448-page Mueller report did not conclude that there was a criminal conspiracy between Moscow and the Trump campaign to influence the 2016 US presidential election.Read More – Source