The US justice department has launched a criminal investigation into the origins of the Mueller inquiry.
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.
The investigation into Russian interference in the 2016 election did not establish any criminal conspiracy between Moscow and the Trump campaign.
But it did not clear the president of obstructing justice. President Donald Trump has long dismissed Robert Mueller's investigation as a "witch hunt".
Reports of the justice department's new criminal inquiry first appeared in the New York Times. It is unclear what potential crime is under investigation, the newspaper said.
Why is the Mueller report being investigated?
The administrative review of the Mueller investigation began in May. It is being overseen by US Attorney General William Barr and is run by US federal prosecutor John Durham.
Mr Durham will have the power to summon witnesses and documents and to enrol a grand jury that could bring criminal charges.
He was tasked with determining whether the collection of intelligence on the Trump campaign in 2016 was lawful.
Mr Durham is widely respected and known for investigating links between FBI agents and organised crime, and investigating the destruction of CIA interrogation videos.
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."
Critics accused Mr Barr of launching an administrative review more in the interests of the president than the interests of justice.
In a joint statement, the chairs of the House judiciary and intelligence committees said reports of a criminal investigation "raise profound new concerns that the Department of Justice under AG Barr has lost its independence and become a vehicle for President Trump's political revenge".
The two Democrats, Jerry Nadler and Adam Schiff, said the move could bring "new and irreparable damage" to the rule of law.
These reports, if true, raise profound new concerns that the Department of Justice under AG Barr has lost its independence and become a vehicle for President Trump’s political revenge. https://t.co/0xxZ8se4Yp via @nytimes— (((Rep. Nadler))) (@RepJerryNadler) October 25, 2019
President Trump said at the time he did not order Mr Barr to launch the administrative review, but added that he was "so proud of our attorney general" and it was "a great thing that he did".
Mr Trump has previously accused the FBI investigators who first launched the probe into his election campaign of treason.
On Friday, Mr Trump told reporters of the investigation: "I think you're going to see a lot of really bad things."
The president said he would "leave it all up to the attorney general".
"I will say this...this was the worst hoax in the history of our country."
What don't we know?
So far, the justice department has not made clear what potential crime is under investigation.
It is also unclear why this investigation has started now, or what prompted it.
And given the department itself appointed Robert Mueller to investigate Russian interference in the 2016 election, this criminal probe means the department may be in essence investigating itself.
What's the Mueller report?
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 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 the hacking of Democratic Party servers by Russian military intelligence, the report said.
Timeline of key events in Mueller inquiry
- September 2016: House Intelligence Committee members announce that Russia is making "a serious and concerted effort to influence the US election"
- May 2017: US Deputy Attorney General appoints Robert Mueller as special counsel to investigate Russian interference in the 2016 election and any alleged coordination with the Trump campaign
- December 2017: Ex-national security adviser Michael Flynn admits lying to the FBI about meetings with Russia's ambassador weeks before Mr Trump became president
- August 2018: Former Trump campaign chief Paul Manafort is found guilty of fraud. He agrees to co-operate with the Mueller inquiry the following month
- November 2018: President Trump submits written answers to the special counsel's questions, days after firing Attorney General Jeff Sessions
- December 2018: Mr Trump's former personal lawyer Michael Cohen is sentenced to 36 months in prison for lying to Congress, tax evasion and campaign finance violations
- March 2019: Attorney General William Barr receives the special counsel's report. He does not release it immediately, but publishes a summary Robert Mueller criticises as lacking "context"
- May 2019: Robert Mueller says in his first public remarks that his report does not exonerate President Trump, and announces his resignation
- July 2019: Mueller appears before Congress and reaffirms that his report does not exonerate Mr Trump