US Attorney General William Barr has assigned a senior federal prosecutor to examine the origins of the Russia investigation, media reports say.
John Durham, the US attorney in Connecticut, has been asked to determine whether the collection of intelligence on the Trump campaign was lawful, the Associated Press reported.
President Donald Trump has long called for such an investigation.
He has branded the Russia inquiry a “witch hunt” by his opponents.
Critics of Mr Barr, however, have accused him of acting on behalf of the president rather than in the interests of US justice.
Last month, the attorney general 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.”
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He gave no details but said he was gathering a team to look into the origins of the investigation, which was led by special counsel Robert Mueller.
Mr Mueller’s 448-page report did not conclude that there was a criminal conspiracy between Moscow and the Trump campaign to influence the 2016 US presidential election.
However, Mr Mueller did detail 10 instances where Mr 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 report said.
Mr Trump and his supporters have accused the FBI and the Justice Department of illegally monitoring his election campaign.
Earlier this month, a US House of Representatives panel voted to hold Mr Barr in contempt of Congress for not releasing an unredacted copy of the report.
The judiciary committee took the rare step as tensions rose over Mr Mueller’s findings.
US Speaker Nancy Pelosi accused Mr Barr of lying to Congress but a justice department spokeswoman called it a “baseless attack”.