WASHINGTON — A review launched by Attorney General William Barr into the origins of the Russia investigation has expanded significantly amid concerns about whether the probe has any legal or factual basis, multiple current and former officials told NBC News.

The prosecutor conducting the review, Connecticut U.S. Attorney John Durham, has expressed his intent to interview a number of current and former intelligence officials involved in examining Russia's effort to interfere in the 2016 presidential election, including former CIA Director John Brennan and former director of national intelligence James Clapper, Brennan told NBC News.

Durham has also requested to talk to CIA analysts involved in the intelligence assessment of Russia's activities, prompting some of them to hire lawyers, according to three former CIA officials familiar with the matter. And there is tension between the CIA and the Justice Department over what classified documents Durham can examine, two people familiar with the matter said.

John Durham speaks to reporters on the steps of U.S. District Court in New Haven, Conn on April 25, 2006. Bob Child / AP file

With Barr's approval, Durham has expanded his staff and the timeframe under scrutiny, according to a law enforcement official directly familiar with the matter. And he is now looking into conduct past Donald Trump's inauguration in January 2017, a Trump administration official said.

Although the probe did not begin as a criminal investigation, Justice Department officials won't comment on whether it has morphed into one.

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When White House chief of staff Mick Mulvaney sought Thursday to justify President Donald Trump's efforts to pressure Ukraine, he called the Durham review "an ongoing investigation by our Department of Justice into the 2016 election."

Mulvaney added: "So you're saying the president of the United States, the chief law enforcement person, cannot ask somebody to cooperate with an ongoing public investigation into wrongdoing?" Mulvaney said.

The FBI began investigating Russian election interference in July 2016. The firing of FBI Director James Comey led to the appointment of Robert 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.

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.

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.

In a May 31 interview with CBS News, Barr said Horowitz "doesn't have the power to compel testimony, he doesn't have the power really to investigate beyond the current cast of characters at the Department of Justice. His ability to get information from former officials or from other agencies outside the department is very limited."

Barr also said in the interview that the purpose of the Durham review is to make sure there was not inappropriate surveillance of the Trump campaign.

"I had a lot of questions about what was going on," he said. "Some of the facts that I've learned don't hang together with the official explanations of what happened."

He declined to elaborate.

President Donald Trump and U.S. Attorney General William Barr attend the 38th Annual National Peace Officers Memorial Service on Capitol Hill on May 15, 2019. Carlos Barria / Reuters file

If Durham is conducting a criminal investigation, it's not clear what allegations of wrongdoing are being examined. The Justice Department has not detailed any, and a spokeswoman declined to comment for this story.

"I don't know what the legal basis for this is," Brennan said, calling the probe "bizarre."

Greg Brower, former assistant FBI director for congressional affairs and a former U.S. Attorney, said it's not normal for a U.S. Attorney to conduct an investigation without an FBI referral of criminal allegations. That's the role of inspector generals, he said.

"It's unusual to the point that it looks to be political and it's a bad thing for DOJ to appear to be doing something for political reasons," Brower said, adding that it appears that "for political purposes, the White House wants to be able to say through the election cycle that all of this is being investigated."

Justice Department officials have said that Durham has found something significant, and that critics should be careful.

Skeptics who have been trying to track Durham's movements say he has yet to interview key figures, including former deputy FBI director Andrew McCabe, former FBI agent Peter Stzrok and former FBI general counsel James Baker.

"Nobody who knows anything has been interviewed," said a person in touch with those former officials.

The New York Times on Saturday reported that Durham and his team appeared to be hunting for signs of anti-Trump bias among former FBI officials and were focused on the actions of Strzok, who was removed from the investigation after Mueller learned of texts he wrote criticizing Trump.

But Durham has been busy on other fronts. He traveled with Barr to the United Kingdom and Italy in an effort to examine the contributions of foreign countries to the Russia probe. A Justice Department spokeswoman said the list of countries being examined includes Ukraine, but she declined to say whether Durham is investigating "corruption related to the DNC server," as Mulvaney put it in his Thursday news conference.

Mulvaney appeared to be referencing a conspiracy theory, mentioned by President Trump in his July phone call with the Ukrainian president, that a Democratic National Committee computer server hacked by Russian intelligence agents is actually in Ukraine.

Acting White House Chief of Staff Mick Mulvaney addresses reporters during a news briefing at the White House on Oct. 17, 2019. Leah Millis / Reuters

Under that discredited theory, Ukraine, not Russia, hacked the Democrats in 2016. To believe that, one would have to doubt the unanimous assessment of the intelligence community and the findings of Congressional intelligence committees who have examined the classified evidence, including Republican Trump supporters.

President Trump's first homeland security adviser, Tom Bossert, said on ABC last month that he was frustrated about the president's embrace of that falsehood.

"It's not only a conspiracy, it is completely debunked," Bossert said. "And at this point I am deeply frustrated with what he and the legal team is doing and repeating that debunked theory to the president. It sticks in his mind when he hears it over and over again and for clarity here ... let me just again repeat that it has no validity."

He continued: "United States government reached its conclusion on attributing to Russia the DNC hack in 2016 before it even communicated it to the FBI, long before the FBI ever knocked on the door at the DNC. So a server inside the DNC was not relevant to our determination to the attribution. It was made up front and beforehand."

A Western intelligence official familiar with what Durham has been asking of foreign officials says his inquiries track closely with the questions raised about the Russia investigation in right-wing media.

Many of those questions spring from accusations made by George Papadopoulos, a former Trump campaign aide who pleaded guilty to lying to the FBI. He declined to comment Friday.

Contrite in the courtroom, Papadopoulos emerged from prison on the attack against what he calls a Deep State conspiracy to set him up in an effort to get Trump.

Papadopoulos claims in his book that the professor who told him the Russians had Democratic emails – Joseph Mifsud, identified by the FBI as a Russian intelligence asset – was in fact working for Western intelligence.

He has made similar allegations against Australian diplomat Alexander Downer, who relayed the story about the emails to the FBI; and about a Cambridge professor, Stefan Halper, whom Papadopoulos says grilled him about what he knew about the Russians.

The New York Times and the Washington Post have reported that Halper is a US government informant, and Durham appears to be investigating whether the spy agency or the FBI broke any rules in how the matter and other aspects of the case were handled.

The Senate intelligence committee examined the allegations about Downer, Mifsud and Halper, as part of its bipartisan investigation into the intelligence community's assessment that Russia was responsible for attacking the 2016 election, and found nothing to substantiate any wrongdoing, a committee aide said.

"I'm very comfortable with everything I was involved in," Brennan said.