A number of former high-level Obama administration officials could fall into the investigative crosshairs of U.S. Attorney John Durham's team as his probe into the handling of the Russia investigation rises to the level of a criminal inquiry.

The elevation in status means the U.S. attorney for Connecticut will be able to subpoena witnesses, file charges and impanel grand juries.

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“You do not impanel a grand jury at this point unless you are going to indict,” a source familiar with the investigation told Fox News. “Durham is at a point where he knows he has crimes and now the question is how many people were involved and they have a pretty good idea of that group of people and what the charges can be and whether or not they can get some cooperators.”

The development has prompted allegations from Democrats that the Department of Justice is being politicized. Attorney General Bill Barr, however, rejected those claims and defended the Durham probe in an interview earlier this week with Fox News, while accusing the James Comey-era FBI brass of a "failure of leadership."

Those bureau leaders and others could well be questioned as part of Durham's probe, as speculation runs rampant over who might be at risk of being charged.

For his part, Comey was asked about Durham’s investigation at the recent Politicon convention in Nashville. He said he's “not worried about a single thing in connection with any of the matters under investigation.”

“Gather the facts, write a report and share it with the American people — please do that,” Comey said. “Don’t drip it out. Don’t leak it out. Give it out. And I’m confident that when the American people see the picture of why we did what we did, their confidence in the institution will be maintained, restored and protected.”

Comey also praised Durham as someone who has “a strong professional reputation” and someone he has “for years thought was an excellent prosecutor."

“I would hope Mr. Durham will do everything possible to protect his reputation from being damaged by those in leadership, and the most important way he can do that is give us transparency,” he continued.

One source told Fox News that Justice Department Inspector General Michael Horowitz’s forthcoming report, which will focus on alleged abuses of the Foreign Intelligence Surveillance Act (FISA) in connection with the early stages of the Russia probe, will shed light on why Durham’s investigation has become a criminal inquiry.

Horowitz, for more than a year and a half, has been investigating alleged misconduct related to the FISA warrants delivered by the Foreign Intelligence Surveillance Court. The Justice Department and the FBI obtained warrants during the final months of the Obama administration in 2016 to surveil Trump adviser Carter Page. It is unclear, at this point, if Page was the only Trump campaign official that the DOJ obtained a FISA warrant against.

With regard to Page’s FISA application, there was “one initial FISA warrant” targeting his and three other renewals from the FISC. The statute requires that every 90 days a FISA order on an American citizen “must be reviewed.”

Former FBI Director Comey signed three FISA applications for Page, while former FBI Deputy Director Andrew McCabe, former Deputy Attorney General Rod Rosenstein, former Deputy Attorney General Sally Yates and former Acting Deputy Attorney General Dana Boente signed at least one, according to a House Republican memo filed last year.

One source told Fox News that it would be a “necessity” for Durham to interview the majority of those officials as part of his probe.

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Meanwhile, Fox News has learned that Durham is “very interested” in questioning former CIA Director John Brennan and former Director of National Intelligence James Clapper. According to a source familiar with the situation, Brennan has received word from his attorney that he may be contacted by Durham’s office, but Clapper has received no such communication at this time.

A prominent Trump critic, Brennan continues to take swipes at the president amid the impeachment inquiry over allegations he improperly pressured Ukraine to launch politically helpful investigations. After Trump questioned the account of a witness who this week voiced concerns about those conversations, Brennan tweeted: "As in previous times of National peril, we rely on our military, diplomats, intelligence officials, law enforcement officers, & other courageous patriots to protect our liberties, freedom, & democracy. May they stay resolute & strong despite corrupt political headwinds they face."

Because Durham’s investigation is focused on a timeline spanning from the weeks leading up to the 2016 presidential election through the spring of 2017, former FBI special agent Peter Strzok will likely be seen as another key figure. Strzok, on July 31, 2016, officially opened the FBI’s initial Russia investigation.

In a separate review led by Horowitz, politically charged text messages between Strzok and former FBI lawyer Lisa Page were revealed. Page was also involved in the early stages of the bureau’s Russia probe, and she and Strzok both later served on former Special Counsel Robert Mueller’s team to investigate Russian meddling and alleged collusion with members of the Trump campaign during the 2016 election.

Strzok's role has also come under question regarding the original interview of former national security adviser Mike Flynn, who later pleaded guilty to giving false statements concerning his discussions with Russia's ambassador. His lawyer recently alleged that official FBI files on that interview were manipulated.

Prosecutors have cast doubt on these allegations, though, saying in a Tuesday filing: "In an extraordinary reversal, the defendant now claims that he is innocent of the criminal charge in this case" and "makes this claim despite having admitted his guilt, under oath, before two federal judges."

Also, as part of his probe, Durham has traveled with Attorney General Barr to Italy and had conversations with law enforcement officials in the U.K. and Australia about their investigation, according to multiple sources familiar with the meetings.

The team is “gathering information from numerous sources, including a number of foreign countries,” Justice Department spokeswoman Kerri Kupec said.

This week, during an exclusive interview with Fox News, Barr said that while he’s assisting in connecting Durham with countries that could have valuable information, Durham is running the show.

“He is in charge of the investigation, I’m not doing the investigation,” Barr said, calling Durham “thorough and fair.”

“Some of the countries that John Durham thought might have some information that would be helpful to the investigation wanted preliminarily to talk to me about the scope of the investigation, the nature of the investigation, and how I intended to handle confidential information and so forth,” Barr said. “So I initially discussed these matters with those countries and introduced them to John Durham and established a channel by which Mr. Durham can obtain assistance from those countries.”

But Democrats have blasted Durham’s investigation, and taken direct aim at Barr.

“The Department of Justice under AG Barr has lost its independence and become a vehicle for President Trump’s political revenge,” House Judiciary Committee Chairman Jerrold Nadler, D-N.Y., and House Intelligence Committee Chairman Adam Schiff, D-Calif., said in a joint statement after learning Durham’s probe had become a criminal inquiry. “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.”

Democratic Sen. Mark Warner also spoke out against Durham’s probe, saying Friday that the Senate Intelligence Committee “is wrapping up a three-year bipartisan investigation, and we’ve found nothing remotely justifying this.”

He called on Barr to testify before Congress.

In response to such criticism, Barr said, "It wouldn't be appropriate at this stage for me to discuss the Durham investigation." He said he'd "certainly inform the public and Congress" when possible.

As for the direction of the investigation, he said: "We’ll let the chips fall where they may.”

Fox News' Jake Gibson and Sam Dorman contributed to this report.