Reports on the criminal probe first appeared in the New York Times late Thursday. It was however, unclear what potential crime was under investigation, the BBC quoted the newspaper as saying.
The administrative review of the Mueller investigation began in May. It was being overseen by the US Attorney-General William Barr and 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.
In 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."
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.
However, it did detail 10 instances where Trump possibly attempted to impede the investigation.
The report concluded that Russia had interfered in the election "in sweeping and systematic fashion".
That interference took the form of an extensive social media campaign and hacking into Democratic Party servers by Russian military intelligence, the BBC reported citing the report as saying.
( With inputs from IANS )