Why has AG Barr enlisted Italy and Australia to review the origins of the Russia probe?

As President Donald Trump last week defended his prodding of Ukraine’s president to dig into still-unfounded allegations against former Vice President Joe Biden, Attorney General William Barr quietly flew to Italy to dig into another set of politically-charged allegations pushed by Trump: allegations that the FBI improperly targeted members of Trump’s presidential campaign during its probe of Russian interference in the 2016 election.

In fact, according to a Justice Department official, the trip to Italy was organized with help from Trump himself, who introduced Barr to the Italian prime minister and other “appropriate officials,” as a Justice Department spokeswoman described them.

Among the others contacted by Trump on Barr’s behalf was the prime minister of Australia, the Justice Department official told ABC News.

What do Italy and Australia have to do with Russian interference in the 2016 election and members of Trump’s presidential campaign? A lot, actually.

At the heart of Barr's inquiry is whether the FBI or any other U.S. agency abused its authority in investigating and conducting surveillance of certain members of Trump's campaign.

In early 2016, Trump’s campaign recruited a new adviser: businessman Carter Page, who -- by his own admission -- had been targeted by Russian spies for recruitment years earlier. After being announced as a member of Trump’s campaign team, Page flew to Moscow to give the commencement address at a prominent graduate school that U.S. authorities have said has ties to Kremlin officials.

Then, in late July 2016, the FBI received what FBI officials at the time described as a startling tip from the Australian government: The Russians had obtained "dirt" on Hillary Clinton and were willing to help the Trump campaign defeat Hillary Clinton.

The tip was passed through another Trump campaign adviser, George Papadopoulos.

A few months earlier, looking to promote his work outside the campaign, Papadopoulos flew to Rome, Italy, where he was introduced by an unidentified individual to Joseph Mifsud, a Maltese-born professor with ties to Russia, according to Special Counsel Robert Mueller’s final report.

“Mifsud at first seemed uninterested in Papadopoulos when they met in Rome,” Papadopoulos told Mueller’s team, the report recounted. “After Papadopoulos informed Mifsud about his role in the Trump Campaign, however, Mifsud appeared to take greater interest in Papadopoulos.”

The two even met a week later in London, where Mifsud “told Papadopoulos that the Russian government had ‘dirt’ on Hillary Clinton in the form of thousands of emails,” according to Mueller’s report and court documents signed by Papadopoulos.

Within a week, Papadopoulos relayed that big claim to another one of his contacts in London: Australia’s top diplomat, Alexander Downer. In fact, Papadopoulos even “suggested” to Downer “that the Trump Campaign had received indications from the Russian government that it could assist the Campaign through the anonymous release of information damaging to candidate Clinton,” according to Mueller’s findings.

The Australian government then alerted the FBI. And, “The FBI opened its investigation of potential coordination between Russia and the Trump Campaign a few days later based on [that] information,” according to Mueller’s report.

In particular, former FBI officials told lawmakers, the FBI wanted to identify who inside the Trump campaign could be in a position to work -- even unwittingly -- with Russian intelligence services.

As the FBI saw it, Carter Page, who was previously targeted for recruitment by Russian spies, was a distinct possibility. And -- in addition to Mifsud’s meeting with Papadopoulos in Rome -- a subsequent allegation against Page relayed to the FBI in Rome is likely part of why Barr is now so interested in Italy.

Even before counterintelligence agents at FBI headquarters in Washington received Downer’s tip alleging Russian efforts to help Trump’s campaign, an FBI agent in Rome received a copy of the so-called “dossier” alleging an even deeper conspiracy between Russia and Trump’s associates.

That document was compiled by former British spy Christopher Steele, a previously-credible source of intelligence for the FBI who sent his salacious and unverified reports to an FBI agent in Rome he knew from such previous work.

The Rome-based agent then forwarded those reports to the FBI’s New York field office in early July 2016, but the reports didn’t make their way to the Washington-based FBI officials investigating Russia’s efforts to interfere in the 2016 election until mid-September, according to congressional testimony from several former FBI officials.

Among other allegations, the reports claimed Page had secretly met with senior Russian officials and that he was told the Kremlin might give the Trump campaign “a dossier of ‘kompromat’” about Hillary Clinton.

While Steele’s reports didn’t prompt the opening of the FBI’s investigation, they did play a substantial part in the FBI later obtaining a federal judge’s approval to eavesdrop on Page’s communications.

That secret surveillance -- and whether it was appropriately “predicated” -- is now a central part of Barr’s review, the attorney general has said.

Mueller's investigation ultimately "did not establish" that Page or anyone else on Trump's team "coordinated with the Russian government in its efforts to interfere with the 2016 presidential election," Mueller's report said.

"I think spying did occur," Barr told lawmakers in April.

The Justice Department’s inspector general, Michael Horowitz, has conducted a similar review, and Horowitz’s office recently completed a draft of its report on the matter, the inspector general recently told lawmakers.

ABC News' Alexander Mallin contributed to this report.