President Donald Trump has repeatedly called the Russia investigation conducted by former special counsel Robert Mueller a "witch hunt."
On Friday, Trump, talking to reporters on the White House lawn, predicted the investigation could bring to light "a lot of really bad things."
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"It looks like it's becoming very serious from what I'm hearing. Investigate the investigators," Trump said.
Republicans have suggested the investigation stemmed from a plot by members of the Obama administration and career intelligence officials, in what they call the "deep state," to undermine Trump.
The FBI began investigating Russian election interference in July 2016. The firing of FBI Director James Comey led to the appointment of Mueller as special counsel in May 2017.
Ultimately, Mueller did not establish that the Trump campaign coordinated with the Russian election interference effort, although he documented dozens of what critics say were inappropriate contacts between Trump aides and Russians.
Michael Horowitz, the Justice Department inspector general, has conducted an investigation into the FBI's actions in launching the Russia probe.
But Barr has said he believes an IG inquiry is not sufficient to answer the questions he has about how the investigation began. In doing so, he made comments suggesting Durham had authority only a criminal investigation could provide.
Barr was criticized after he released a brief account of the Mueller report in March that critics said was inaccurate. Mueller, in a letter and phone call, said that Barr's four-page description of what Barr called the report’s principal conclusions did not fully capture the context and substance of the more than 440-page document.
That the administrative review into the origins of the Mueller probe has turned into a criminal investigation could raise alarms that Trump is using the Justice Department to go after his perceived enemies, the Times reported.
Democrats in the House are conducting an impeachment inquiry into Trump that centers on alleged attempts to pressure the president of Ukraine to announce investigations into what has been described as a conspiracy theory about interference in the 2016 election, as well as into former vice president Joe Biden and his son, Hunter.
Critics say that alleged pressure amounts to Trump using withheld foreign aid and the power of the presidency to advance his own political interests, inviting a foreign power to again interfere with the U.S. presidential election.
Democratic Reps. Adam Schiff, who is chairman of the House Intelligence Committee, and Jerrold Nadler, who heads the House Judiciary Committee, called the reports of a criminal investigation troubling.
"These reports, if true, raise profound new concerns that the Department of Justice under Attorney General William Barr has lost its independence and become a vehicle for President Trump's political revenge," the lawmakers said in a statement.
"If the Department of Justice may be used as a tool of political retribution or to help the President with a political narrative for the next election, the rule of law will suffer new and irreparable damage,” they added.
Julia Ainsley is a correspondent covering the Department of Homeland Security and the Department of Justice for the NBC News Investigative Unit.