Christopher Steele, the author of the 'dirty dossier' on Donald Trump, was interviewed by Justice Department inspector general officials for 16 hours - and found to be credible as part of their probe into whether the FBI and DOJ abused surveillance powers against one of the president's campaign advisers.

The interview with Steele, a former top spy on Russia for Britain's Secret Intelligence Service, or MI6, took place while Trump was in London in June for a formal state visit with Queen Elizabeth and a meeting with UK Prime Minister Theresa May.

It was part of an inquiry being conducted by the DOJ inspector general into the use of FISA surveillance powers by the FBI - then led by James Comey - and DOJ before the 2016 election, which targeted Trump campaign worker Carter Page.

The inspector general is investigating whether the 35-page 'golden showers' dossier drawn up by Steele and paid for ultimately by Hillary Clinton's campaign may have been improperly used by officials to gain surveillance warrants targeting the two, and possibly others.

Separately, Attorney General William Barr started a probe into how Mueller's investigation began - a move that thrilled President Trump who has long claimed he was the victim of a 'witch hunt.' Steele is not known to have been interviewed in that probe, which is being led by John Durham, U.S. attorney for Connecticut.

Christopher Steele was found to be credible during a 16-hour interview by Justice Department officials investigating the origins of Mueller's Russia probe

Steele was interviewed in June when President Trump met with the Queen during a state visit

Three attorneys from the Inspector General's office of the U.S. Department of Justice met in person in early June with Steele in Britain, said two sources with direct knowledge of the lawyers' travels.

The interview - held over more than one day - lasted 16 hours, sources told Politico.

Steele's dossier, made public in 2017, alleged that Moscow attempted to interfere in the 2016 U.S. presidential election and that there was potential collusion between Russia and Trump's campaign, along with other unverified and salacious claims about the president, including that he was vulnerable to blackmail from the Russians.

The Justice Department's inspector general has been examining the earliest stages of an FBI investigation of Trump, his former 2016 presidential campaign rival Democratic nominee Hillary Clinton, Russia and former Trump adviser Carter Page.

Inspector General Michael Horowitz, whose office is an internal Justice Department watchdog, launched his probe in March 2018 amid allegations by Republican lawmakers that the FBI erred in seeking a warrant to monitor Page.

Target: Carter Page, a Trump campaign adviser, was the subject of a surveillance warrant. The DOJ inspector general is investigating whether there was abuse of the FISA surveillance powers to gain the warrant

At the time it was granted by the FISA court, James Comey was FBI Director, and other officials involved in the process included his deputy Andrew McCabe, counter-intelligence chief Peter Strzok, and his lover, bureau attorney Lisa Page.

Then assistant attorney general Bruce Ohr was also involved in the application process. All have been frequent targets for Trump's anger since he fired them.

The court which oversees FISA - the Foreign Intelligence Surveillance Act - can grant warrants for surveillance on an American suspected to be an agent of a foreign power, whether wittingly or not.

When the FBI and DOJ apply for warrants to use surveillance powers against American targets, they have to disclose the reasons for their suspicion, and include any exculpatory information.

Republican claims of misconduct center on whether the Steele dossier was used in the application without the judges being told it was commissioned by opposition research group Fusion GPS, who in turn were being paid by the Clinton campaign's attorneys, Perkins Coie.

Fusion GPS was at first commissioned to look into Trump during the Republican primaries.

The company was paid by the Washington Free Beacon, the conservative news website, which is supported by then anti-Trump Republican Paul Singer, with the intention of information going to the candidate's opponents in his own party.

After Trump won the nomination, Fusion GPS was paid to continue its work by Perkis Coie, and in May 2016 commissioned Steele.

The New York Times reported in April that Steele has told acquaintances he did not know that the ultimate client he was working for was the Clinton campaign.

Trump has described the Steele dossier as 'bogus' and Republicans have long sought to discredit the FBI's investigation, which was later taken over by Mueller.

His final report on Russia and the Trump campaign was released in redacted form in mid-April.

In that same month, Barr told a congressional committee that the Horowitz probe would be completed by May or June.

That is now clearly not the case. One of the two sources said Horowitz's investigators appear to have found Steele's information sufficiently credible that they had to extend the investigation. Its completion date is now unclear.

Horowitz's team has been gauging Steele's credibility as a source for the FBI.

The interview was contentious at first, sources told Politico, but ultimately, the DoJ lawyers found Steele's testimony credible and even surprising.

A key focus of the Horowitz probe is whether the FBI followed proper procedures when it applied for a warrant with the Foreign Intelligence Surveillance Court (FISA) to secretly conduct surveillance on Page and his ties to Russia.

Declassified documents show that the FBI cited information in Steele's dossier when it asked the secrecy-wrapped FISA court in late 2016 for a warrant to eavesdrop electronically on Page, a U.S. businessman with interests in Russia.

Attorney General William Barr ordered the investigation into the origins of the Russia probe

The Justice Department is investigating how special counsel Robert Mueller's probe began

If Steele is found to be credible by the president's own Justice Department's inspector general, that could throw cold water on the argument by Trump allies that Steele's dossier was insufficient to prompt the FBI's actions.

Steele has long said his dossier was not used to start the FBI's counter-intelligence investigation into Trump's campaign in the 2016 election.

Its explosive claims that Trump ordered prostitutes to urinate on a Moscow hotel bed because it was once slept in by Barack Obama became public shortly after he was briefed on the existence of the claims by then FIB director James Comey.

Horowitz's office in Washington declined to comment.

A spokesman for Orbis Business Intelligence, Steele's London investigations firm, declined to comment.

UK government agencies declined to disclose whether they were in contact with Horowitz's team in London.

Moscow repeatedly rejected accusations of interfering in the election.

Mueller's report, released on April 18, said that Russia did meddle in the election in an attempt to boost Trump's candidacy. It said Trump campaign officials had multiple contacts with Russian officials. But it found insufficient evidence to establish a criminal conspiracy between the campaign and Moscow.

The report also described numerous attempts by Trump to impede Mueller's inquiry, but it stopped short of declaring that he committed a crime. Mueller during his inquiry brought charges against 34 people, including Russian agents and ex-Trump aides.

Page, a foreign policy adviser during Trump's campaign, drew scrutiny from the FBI, which said in legal filings in 2016 that it believed he had been 'collaborating and conspiring' with the Kremlin. But he was not charged.

Investigators working for Mueller in September 2017 twice interviewed Steele, who also gave written testimony to the U.S. Senate Intelligence Committee in August 2018. That panel and other committees of Congress have taken up where Mueller left off after his investigation ended.

Ahead of Trump's state visit, a U.K. official said the country would be 'helpful' during the DoJ probe.

'If we get that request obviously we'll consider it. The U.S. is an ally and we would want to be helpful,' the official said.