Washington (CNN) Federal prosecutor John Durham, known for bringing independent rigor to challenging and politically sensitive cases, just got his latest high-profile assignment.

His role in Barr's controversial review has earned bipartisan praise. Trump's staunchest allies heralded the addition of a battle-tested investigator to Barr's efforts, while some prominent Democrats say they are confident Durham will honestly follow the facts and get to the truth.

CNN reported Monday that Durham has already been working on the internal review for weeks, according to a source familiar with the matter. Barr's team is getting assistance from the FBI and CIA , but the review hasn't reached the level of a criminal investigation.

In the past week, former Director of National Intelligence James Clapper , former FBI Director James Comey , former Deputy Attorney General Rod Rosenstein and former top FBI lawyer James Baker all publicly defended the decision to open the Russia probe in 2016, justifying the legitimacy of the probe in separate comments.

Handling the most challenging cases

Durham, 69, is a registered Republican, according to public listings, and it doesn't appear that he ever donated to any political candidates, according to Federal Election Commission records. He has been a Justice Department employee under six American presidents, and has worked for the government -- both state and federal -- ever since he became a lawyer in 1975.

During his 37-year career at the Justice Department, Durham worked his way up the ranks and has handed some of the most sensitive investigations in recent memory. He was entrusted with such high-profile responsibilities in both Democratic and Republican administrations.

In 1999, then-Attorney General Janet Reno appointed Durham to investigate corruption by law enforcement in Boston. His investigation looked at how the FBI used informants to infiltrate organized crime, and he successfully prosecuted an FBI supervisor and a police lieutenant.

During the George W. Bush administration, then-Attorney General Michael Mukasey selected Durham to investigate the CIA for destroying videotapes of interrogations that included torture techniques. The investigation stretched into the Obama administration when then-Attorney General Eric Holder expanded Durham's mandate. Durham closed the probe without bringing any charges.

"He's been tasked with sensitive, significant and complex investigations on a number of occasions, during Democratic and Republican administrations," said former US attorney Deirdre Daly, who preceded Durham and was appointed by President Barack Obama. "That should give the public confidence that he'll approach this in a fair, balanced and appropriate way."

"John is tireless, fair and aggressive," Daly, who overlapped with Durham for seven years in the US attorney's office, added.

A spokesman for the US attorney's office declined to comment for this article.

In addition to his work as a special prosecutor, Durham has extensive experience as a trial lawyer. He went after gangs in New Haven, prosecuted mob figures, and secured a guilty plea from former Connecticut Gov. John Rowland, a Republican who admitted to corruption.

He graduated from the University of Connecticut School of Law and received his undergraduate degree from Colgate University. He graduated with honors, according to his official biography

His first job was with VISTA, now known as AmeriCorps, where he worked as a lawyer in Montana and provided legal services to the Crow Indian Tribe and its members. His work there largely focused on issues regarding land use and natural resources, he told lawmakers.

Durham later returned to Connecticut and became an assistant state's attorney. He joined the Justice Department in 1982, launching a nearly four-decade federal career. Last year, the Senate unanimously confirmed him to become the top federal prosecutor in Connecticut.

A fierce prosecutor who shuns the spotlight

It's difficult to find pictures of Durham on the internet. One image that has widely circulated this week, of Durham standing in front of the New Haven courthouse, is 13 years old.

News conferences and public interviews have been rare for Durham during his career. In a written questionnaire as part of his Senate confirmation process, Durham said he didn't have records of any interviews with newspapers, magazines, radio stations or television networks.

"I would be very surprised if there were any leaks from John or anyone on his team," Daly said. "I think he will do his work quietly and efficiently, but outside of the public eye."

Boston Globe article from January 2008 noted that Durham was known for his "tenacity" as a prosecutor in tough cases. But the article also said Durham was famous in New England legal circles "for his avoidance of the media" and that he "seldom comments publicly on cases."

But the newspaper did unearth one quote from a news conference: "Nobody in this country is above the law, an FBI agent or otherwise, and ultimately the ends do not justify the means."

The quote -- one that could reverberate with his new assignment -- came on the day Durham secured the conviction of an FBI supervisor on racketeering and obstruction charges that stemmed from his years-long investigation into FBI corruption in the Boston mob scene.

Bipartisan praise for his 'independent' streak

Trump, who has publicly called for the Justice Department to "investigate the investigators" ever since special counsel Robert Mueller wrapped up his Russia probe, has praised Barr's expansion of the review.

"I think it's a great thing that he did it," Trump said of Barr's decision to get Durham involved in the internal review. "I am so proud of our attorney general that he's looking into it."

Rep. Mark Meadows, the Republican chairman of the hardline Freedom Caucus, tweeted that Durham was an "outstanding choice" and said it demonstrated the "tenacity" or Barr's probe. Meadows is a critic of the Russia investigation and threatened to support impeachment for Justice Department officials who didn't provide documents about the start of the investigation.

Trump campaign aide George Papadopoulos, whose interactions with FBI informants in 2016 are part of Barr's review, also heralded the move. Papadopoulos pleaded guilty in 2017 to lying to the FBI about his Russia contacts, though he now says the entire probe was a set-up.

"It's about time that the origins are investigated," Papadopoulos told CNN on Tuesday. "I am certain that my version of events will end up being the correct ones for the history books. I have full faith in the new team assembled and look forward to assisting them if required."

But some Democrats also are cautiously optimistic about Durham.

Democratic Connecticut Sen. Chris Murphy told CNN that Durham has a reputation for being "apolitical and fair," and noted that he backed Trump's decision to pick Durham as US attorney.

"He's been a prosecutor in Connecticut forever," Murphy said of Durham, who has been a public official in the state since 1977. "He's a law-and-order guy, tough-nosed, well-respected. If I were him, I wouldn't have taken this job, but he's got a reputation of being apolitical and serious."

Still, Murphy said Barr's examination of the Russia probe is "designed to be a political hit-job." He said he hopes Durham "is taking it on with a desire to get to the truth and get there quickly."

Daly, a Democrat, also called Durham "apolitical," adding that "he's independent and will be fair with his direction."