The so-called investigation of the investigators is led by John Durham, a Connecticut-based federal prosecutor, who so far has conducted some interviews but also has run into some obstacles from witnesses who have declined voluntary interviews, CNN reported last week.
The move to make it a criminal inquiry was always anticipated, and it allows Durham to use subpoenas to compel testimony and comes as President Donald Trump faces an onslaught of negative headlines stemming from the House impeachment inquiry into his dealings with Ukraine. It's not clear what, if any, part of the Trump-Russia investigation is a target of Durham's criminal probe.
The New York Times was first to report on the new stage of the investigation.
The investigation has been driven by Barr's suspicions that some of the officials overseeing the counterintelligence probe of the 2016 Trump campaign may have acted improperly.
Barr's embrace of these theories aligns with Trump's chief grievance that he was the victim of a "deep state" spy operation that has clouded his presidency.
The President has publicly called for investigations of former FBI Director James Comey, former CIA Director John Brennan and former Director of National Intelligence James Clapper, among others.
And in recent weeks the President's eagerness for the Justice Department to focus on his perceived critics has caused awkward issues for Durham and the department.
Department officials have said Barr didn't know that Trump had mentioned his name in a July call with Ukraine's President, suggesting he work with the attorney general and Rudy Giuliani, the President's personal attorney, whose activities are near the center of the congressional impeachment inquiry. And a senior Justice official disavowed comments from acting White House chief of staff Mick Mulvaney, who linked a freeze of Ukraine aid to the Justice Department probe.
The probe has been unusual for various reasons, not least because Barr has taken a hands-on role to closely manage it.
He, along with Durham, traveled to countries overseas, including Italy twice, to meet with officials seeking to gather information.
One particular fixation for Barr has been Joseph Mifsud, a shadowy professor whose discussions with a Trump campaign associate became part of Robert Mueller's special counsel investigation, according to a person briefed on the matter.
Mifsud has been a regular fixture in conservative media stories, which claimed he was working for US or Western intelligence and was tasked to spy on the Trump campaign.
Barr has also said that the review would include an examination of former British intelligence officer Christopher Steele's work compiling research about Trump and Russia in a dossier that was commissioned by Fusion GPS, a research and investigative firm.
This story has been updated.