When the U.S. government needed a prosecutor to ferret out corruption in its own law-enforcement and intelligence ranks, John Durham was its go-to guy. The longtime prosecutor helped exonerate men wrongly convicted on murder charges, exposed an FBI agent tied to one of Boston’s most notorious gangsters and dug into the CIA’s destruction of video tapes thought to show foreign detainees being tortured.

But some of Durham’s actions in his latest high-profile assignment — examining the FBI’s 2016 investigation of President Trump’s campaign — have sparked a debate in Washington about whether he is even-handedly assessing possible wrongdoing, or carrying out a conservative political errand.

Last week, after the Justice Department inspector general released a report concluding the bureau had adequate cause to open the investigation into possible coordination between the Trump campaign and Russia, Durham issued a remarkable public statement registering his disagreement.

The move left many inside and outside the Justice Department puzzled, as it seemed out of step with Durham’s character. In general, law enforcement considers it inappropriate to comment on ongoing investigations, and Durham is known for being especially tight-lipped. In sworn testimony before Congress, Inspector General Michael Horowitz later described his and Durham’s dispute as being over a relatively unimportant bureaucratic distinction about how the probe was categorized — a matter that likely would not have had a substantive impact on the Russia case.

“I am bothered because his statement is completely unnecessary and, to me, is violative of the norms and customs of the Justice Department,” said former federal prosecutor Gene Rossi, who now works at the Carlton Fields law firm. “He has an ongoing investigation. He has not concluded it. And what he’s doing is he’s issuing a smoke signal that I may have information that is contrary to the report.”

A spokesman for Durham declined to comment.

Durham’s defenders say he will not be swayed by politics, no matter how intense the partisan firestorm. The president and his allies have long attacked the Russia investigation and pushed aggressively to have those who led it scrutinized. Attorney General William P. Barr assigned Durham to do so earlier this year, even though Horowitz was already conducting a similar review.

Barr told NBC News recently that he expects Durham’s probe to potentially reach a “watershed” in the late spring or early summer, just as the 2020 election campaign heats up.

From the beginning, some in law enforcement and intelligence circles have bristled at Durham’s assignment. Repeatedly investigating the investigators, current and former Justice Department officials said, risks discouraging FBI agents and prosecutors from initiating cases against high-profile targets. The officials said they worried Durham’s probe was meant mainly to validate Trump and Barr’s criticism of the Russia probe.

In a Fox News interview in November, Trump touted the prosecutor’s reputation — though he referred to him as “Bull Durham” — saying he was “supposed to be the toughest.”

“We would maybe have ended this thing a lot sooner had he been there originally,” Trump said.

Law enforcement officials were heartened initially that Durham was chosen for the sensitive task because of his sterling reputation. Durham had been asked several times by attorneys general from both political parties for similarly high-profile assignments.

Former attorney general Janet Reno, for example, tapped Durham in 1999 to investigate possible FBI corruption in Boston. Durham ultimately brought charges against an agent who helped the notorious gangster Whitey Bulger, a bureau informant, avoid justice. The agent, John Connolly, was convicted of murder and racketeering.

Former assistant U.S. attorney Brian T. Kelly, who worked with Durham on the case, said Durham was a “consummate professional” who would not be swayed by partisan concerns in the Russia matter.

“He’ll call balls and strikes as he sees them,” Kelly said.

Former attorney general Michael Mukasey, who asked Durham in 2008 to investigate the CIA’s destruction of video tapes showing detainee interrogations overseas, said he chose Durham because he was “somebody of just complete integrity and discretion.” He said he found Durham’s recent statement was “unusual,” but not inappropriate.

Durham has broader powers than Horowitz, the inspector general. The Trump-appointed U.S. attorney in Connecticut can convene a grand jury to compel witness testimony, and potentially charge people with crimes if he finds evidence to do so.

Horowitz referred one such case to Durham involving an FBI lawyer who was found to have altered a document used as the FBI applied to monitor a former Trump campaign adviser, Carter Page.

Horowitz limited his review to assessing the opening of the Russia case before Trump was elected, the FBI’s use of confidential informants and its secret court applications to surveil Page, who was of interest to investigators because he had met previously with known Russian intelligence operatives. Barr told NBC News that he had instructed Durham to examine FBI actions after Trump’s election and look at agencies outside of the Justice Department.

Durham has been conducting interviews and traveling abroad with Barr to talk to foreign intelligence officials, though it is unclear what his investigation has uncovered. He had not spoken publicly about the probe before issuing his statement disputing the inspector general report.

The report concluded that the FBI had an “authorized purpose” to begin investigating the Trump campaign in July 2016, and that there was no evidence the decision to do so was driven by political bias. Soon after it was released, Barr issued a statement saying he disagreed. Durham’s followed within a half-hour.

“Based on the evidence collected to date, and while our investigation is ongoing, last month we advised the Inspector General that we do not agree with some of the report’s conclusions as to predication and how the FBI case was opened,” Durham said.

During testimony to the Senate Judiciary Committee on Wednesday, Horowitz said he stood by his conclusion and was “surprised” by Durham’s statement.

People familiar with drafts of Horowitz’s report said the document initially contained a footnote saying that the inspector general had asked Durham whether he had evidence to support a theory pushed by the political right that the investigation began as a setup by U.S. intelligence using a Maltese professor to entrap the Trump campaign, and Durham had responded he had not.

Such a footnote about Durham and the origins of the probe was not in Horowitz’s final report, and the people familiar with earlier drafts said it had apparently been removed or redacted. Horowitz declined to address questions about the drafts at the congressional hearing, though he said he was unaware of what Durham might have found that might change his view about the case’s origins.

Horowitz told lawmakers his office had met with Durham in November, and that Durham said he felt the FBI had adequate cause to open a “preliminary investigation,” but not a full one, when it did so in July 2016.

The distinction is a small one that typically has little bearing on the early stages of an investigation. Horowitz said that even if agents had opened a preliminary investigation, rather than a full investigation, they still would have been authorized to use informants as they did in the first month of the case. They would not have been allowed to seek surveillance warrants — though they did not do that until later, in October 2016.

“The controversy that they’re making a kerfuffle over is beyond ridiculous,” said Rossi, the former federal prosecutor. “It’s like arguing over the pronunciation of ‘toe-mae-toe’ versus ‘toe-mah-toe.’ It’s still a red tomato.”

Current and former officials soon began wondering aloud what Durham could have been thinking, and speculating that Barr pressured the respected prosecutor to issue the statement.

Former attorney general Eric Holder, who tapped Durham to expand his examination of the treatment of CIA detainees after his work for Mukasey, wrote in a Washington Post column that he had been “proud to know John for at least a decade,” but was troubled by the prosecutor’s public statement.

“Good reputations are hard-won in the legal profession, but they are fragile; anyone in Durham’s shoes would do well to remember that, in dealing with this administration, many reputations have been irrevocably lost,” Holder wrote.

Gregory A. Brower, who served as a U.S. attorney during the George W. Bush administration and is now a partner at the Brownstein Hyatt Farber Schreck law firm, said: “Everybody who knows Durham has nothing but positive things to say about his experience, integrity, and professionalism, but the incredibly unusual nature of his role, his apparent mandate, and now more recently his public statement about an ongoing investigation, together are causing a lot of former Justice Department officials to question the legitimacy of what he’s doing.”

Barr has defended Durham’s statement, telling NBC News that it was “perfectly appropriate” and designed to avoid public confusion.

“I think it was important for people to understand that Durham’s work was not being preempted, and that Durham was doing something different,” Barr said.

Durham ultimately never charged anyone in connection with the CIA case, and never made public any final report of his work. It is unclear what he plans to do at the conclusion of his Russia investigation.

His supporters say that his work is likely to have an impact. Massachusetts lawyer Victor Garo recalled that when Durham was investigating corruption among FBI agents in Boston in the late ’90s and early 2000s, Durham showed up at his office one December night with a stack of documents. At the time, Garo was representing a man who had been charged and convicted of murder. Garo believed that was in error.

The documents that Durham uncovered, Garo said, proved not only that his client, Joseph Salvati, had been wrongly convicted, but that the FBI had long known who the real killers were. Garo said authorities ultimately dropped all the charges against his client and others wrongly accused. Salvati, who spent nearly 30 years in prison, and others later reached a settlement with the government for more than $100 million.

“If John Durham finds something, you will know about it. If he doesn’t find something, he won’t make up evidence to say it, either,” Garo said. “He is not owned by anybody.”