WASHINGTON (NYTIMES) - A top aide to the criminal prosecutor whom Attorney-General William Barr assigned to scrutinise the Trump-Russia investigation has resigned unexpectedly from the Justice Department, a spokesman said Friday (Sept 11).

It was not immediately clear why the official, Nora R Dannehy - a trusted assistant to John H Durham, the prosecutor leading the investigation and the US attorney in Connecticut - stepped down.

But The Hartford Courant, which first reported her departure, cited unidentified colleagues in Durham's office as saying that she had expressed concerns in recent weeks about pressure from Barr to deliver results ahead of the presidential election in November.

Dannehy did not respond to a voicemail message seeking comment. A spokesman for Barr, Kerri Kupec, referred questions to Durham's office in Connecticut.

His spokesman, Thomas Carson, said: "We can confirm that Nora Dannehy has left the Department of Justice. No further comment from us."

Several officials said expectations had been growing in the White House and Congress that Barr would make public, ahead of the election, some kind of interim report or list of findings from Durham before he completed the investigation.

Barr had wanted Durham's team to move quickly, according to a person familiar with the matter.

Trump has long promoted Durham's investigation as a way to back his own claims that the counterintelligence officials who sought to understand his campaign's ties to Russia were carrying out a deep-state plot to sabotage him.

After the Justice Department's independent inspector-general, Michael E Horowitz, found serious errors last year with one aspect of the Russia investigation but debunked any politically motivated plot, Trump said: "I look forward to the Durham report, which is coming out in the not-too-distant future. It's got its own information, which is this information plus plus plus."

Whether Barr and Durham plan to take public investigative steps close to the election, flouting a long-standing Justice Department practice of avoiding overt activity within 60 days of an election if it could have a political impact on the vote, has been the subject of growing scrutiny.

Barr has repeatedly raised tantalising hints about Durham's work, contrary to the department norm against speaking about continuing investigations except through indictments.

And he has put forward a narrow interpretation of the so-called 60-day rule, saying it only meant the department should not indict a candidate for office or a close campaign associate.

"The idea is you don't go after candidates, you don't indict candidates, or perhaps someone that's officially close to a candidate - that is essentially the same - within a certain number of days before an election," Barr said in an April interview with conservative radio host Hugh Hewitt.

"But, you know, as I say, I don't think any of the people whose actions are under review by Durham fall into that category."

Barr assigned Durham last year to scrutinise the early stages of the Trump-Russia investigation shortly after special counsel Robert Mueller turned in his report that documented Russia's extensive operations to sabotage the 2016 election and Trump's efforts to thwart the inquiry.

Durham had a history of being assigned to lead sensitive investigations of government conduct, including the FBI's ties to a crime boss in Boston and the CIA torture of detainees.

While he is a longtime career prosecutor, Trump gave him a political appointment as the US attorney for Connecticut in 2018.

Dannehy, who successfully prosecuted former Connecticut Governor John G Rowland on corruption charges, had worked closely with Durham for years.

Her husband, Leonard C Boyle, is also a close colleague of Durham's, serving as the first assistant US attorney in Connecticut.

He had previously served as the head of the Terrorist Screening Centre, appointed by Mueller when he was FBI director.

After Barr assigned Durham to investigate the Russia inquiry, he asked Dannehy to return from the private sector to serve as essentially his top investigator on the case.

She has played a leading role in questioning witnesses about investigative actions, according to people familiar with the sessions.

This was not the first time that Dannehy had taken on a high-profile, politically fraught investigation.

After a scandal over the George W Bush administration's firing of US attorneys who were balking at demands, including by the White House, to bring or speed up voter-fraud cases against Democrats ahead of an election, Attorney-General Michael B Mukasey appointed her to scrutinise whether any laws were broken in one such instance.

While Dannehy did not find criminal wrongdoing in that dismissal, she suggested in her report that political pressure to rush out a case or charging decision before an election was wrong and potentially constituted obstruction of justice.

"Pressuring a prosecutor to indict a case more quickly to affect the outcome of an upcoming election could be a corrupt attempt to influence the prosecution in violation of the obstruction of justice statute," Dannehy wrote.

"The same reasoning could apply to pressuring a prosecutor to take partisan political considerations into account in his charging decisions."

Few details about her resignation were disclosed Friday evening, even as associates portrayed her as both a circumspect person and a prosecutor of rectitude.

"Nora Dannehy is apolitical, incorruptible and among the best prosecutors to ever represent the United States," said Christopher M Mattei, former chief of the financial fraud and public corruption unit at the US attorney's office in Connecticut.

Barr's appointment of Durham is part of a broader pattern in which he has repeatedly sought to cast aspersions on the Trump-Russia investigation, including by putting out an early public description of the then-still-secret Mueller report that a federal judge later said was so "distorted" and "misleading" in making it sound better for the president than it was that the judiciary could not trust his department on the topic.

When Horowitz completed his own review of the early stages of the FBI investigation, he concluded that the bureau had a lawful basis to open the counterintelligence inquiry.

In an extraordinary move, the Justice Department stepped on that finding by releasing a statement in the name of Durham disagreeing with Horowitz's conclusion based on his own continuing inquiry, and let it be known that Barr sided with Durham.

To date, the only publicly apparent concrete accomplishment of the Durham inquiry derived from Horowitz's review.

The errors that the inspector-general's team uncovered were in materials submitted for four wiretap applications under the Foreign Intelligence Surveillance Act targeting a former Trump campaign foreign policy adviser with significant links to Russian intelligence officials, Carter Page.

As part of that work, the inspector-general's team uncovered that during the preparations to apply for the final renewal of the wiretap, an FBI lawyer assigned to assist the Crossfire Hurricane team, Kevin E Clinesmith, prevented a colleague from recognising that both that application and the three earlier ones had an important omission by adding words to a CIA email about Page and showing it to the colleague.

Horowitz referred Clinesmith for criminal investigation, and Durham's team prosecuted the case.

Last month, Clinesmith pleaded guilty to one count of making a false statement by doctoring the email.