WASHINGTON — After a jet carrying Attorney General William P. Barr touched down in Rome late last month, some diplomats and intelligence officials at the American Embassy were unsure why he had come to the Eternal City. They were later surprised, two officials said, to discover that he had circumvented protocols in arranging the trip, where he met with Italian political and intelligence officials.
Everything about the visit was unusual — perhaps most of all, the attorney general’s companion and his mission. Mr. Barr and a top federal prosecutor, John H. Durham, who is reviewing the origins of the Russia investigation, sought evidence that might bolster a conspiracy theory long nurtured by President Trump: that some of America’s closest allies plotted with his “deep state” enemies in 2016 to try to prevent him from winning the presidency.
Mr. Trump has embraced the theory in his interactions with world leaders since the days after the special counsel, Robert S. Mueller III, testified to lawmakers in July that his investigation found insufficient evidence to charge any Trump associates with conspiring with Russia to help subvert the election. An emboldened Mr. Trump — who could benefit politically if Mr. Durham were to unearth facts that undermined Mr. Mueller’s investigation — began pressing close allies to cooperate with the review.
The trip to Italy generated criticism that Mr. Barr was doing the president’s bidding and micromanaging a supposedly independent investigation. But Mr. Barr seems to have embraced his role, signaling that he has made the investigation a priority and is personally overseeing it.
Now, glimpses of the Durham review are emerging. Investigators have interviewed F.B.I. officials about their work in 2016, examined intelligence files from around that time and cast a wide net in setting up interviews with a foreign cast of characters who played disparate roles in the pre-election drama.
One of Mr. Trump’s efforts to aid the review, a discussion with President Volodymyr Zelensky of Ukraine one day after Mr. Mueller’s testimony, so unnerved White House officials that it sparked a whistle-blower complaint, as well as formal impeachment proceedings and questions about whether the president hijacked American diplomacy for political gain.
Mr. Barr has portrayed the review as an attempt to ferret out any abuse of power by law enforcement or intelligence officials. But it is also a politically charged effort that takes aim at the conclusions of the American law enforcement and intelligence communities about Russia’s election interference based on years of work by multiple agencies.
The review could fray diplomatic relations with overseas partners and affect Mr. Trump’s political fortunes. And it is testing traditional boundaries drawn to keep the powers of American law enforcement out of electoral politics.
A Justice Department spokeswoman declined to comment. This article is based on documents and interviews with current and former American and foreign officials as well as others familiar with the Durham review.
The review already created a minor diplomatic dust-up when Senator Lindsey Graham, Republican of South Carolina and one of the president’s closest allies in Congress, fired off a letter to leaders of Britain, Italy and Australia on Wednesday, urging them to help “investigate the origins and extent of foreign influence in the 2016 election.”
All three countries play some role in a counternarrative pushed by the president’s supporters that the real story of election sabotage in 2016 was not the well-documented saga of Russian internet trolls and leaked stolen emails, but anti-Trump elements in the intelligence and law enforcement agencies working with sympathetic foreign allies to try to block Mr. Trump’s victory.
Mr. Graham asserted without evidence in his letter that an Australian former diplomat was involved in the supposed plot. Australia’s ambassador to the United States, Joe Hockey, responded sharply, rejecting Mr. Graham’s description of the role of the diplomat, Alexander Downer.
The president further stoked the flames on Friday, suggesting a broad foreign plot against him. “And just so you know — just so you know, I was investigated,” he told reporters on the South Lawn of the White House. “I was investigated. O.K.? Me. Me. I was investigated. I was investigated. And they think it could have been by U.K. They think it could have been by Australia. They think it could have been by Italy.”
He did not say whom he meant by “they.”
One consequence of the president’s attempts to investigate the investigators could be that some American allies might think twice before providing politically sensitive information.
“I’m gravely concerned if our Australian intelligence colleagues believe that they are sharing information with us for domestic political purposes, that trust could erode,” said Senator Mark Warner of Virginia, the top Democrat on the Senate Intelligence Committee.
Mr. Warner said he and his colleagues have pressed the Justice Department for information about the scope of the review but have gotten no response.
The president has handed Mr. Barr sweeping powers to conduct the review. It was not begun as a criminal investigation, though it is not clear whether that has changed. In conducting a review, Mr. Durham, the United States attorney in Connecticut and a veteran prosecutor who has broken up mafia rings and investigated C.I.A. torture, has no power to subpoena witnesses or documents and instead has the authority only to read materials the government already gathered and to request voluntary interviews from witnesses.
Typically, he would write a report at the end of his review summarizing his findings. If he finds evidence of a crime, Mr. Durham could make a criminal referral to the Justice Department.
Mr. Barr has asked Mr. Trump to help gain access to foreign officials for the inquiry, and the president has complied. Mr. Trump has called the leaders of Ukraine and Australia, and the attorney general has spoken directly to officials in Britain, Australia and Italy, according to a Justice Department official.
Mr. Barr and Mr. Durham traveled to Italy — the attorney general’s second trip there in weeks — where a Trump campaign foreign policy adviser, George Papadopoulos, met a Maltese professor in the spring of 2016. During a later meeting, Mr. Papadopoulos told investigators, the professor said that Russia had politically damaging information about Hillary Clinton in the form of “thousands of emails.”
The professor, Joseph Mifsud, has effectively disappeared since the Mueller investigation revealed his discussions with Mr. Papadopoulos, and Mr. Mueller’s prosecutors suggested in a court filing that he may have served as a cutout for Russian intelligence.
Mr. Trump’s allies have asserted, without evidence, that he was actually a C.I.A. agent working as part of an Obama administration plot to spy on the Trump campaign.
“Mifsud was an Italian operative handled by the C.I.A.,” Mr. Papadopoulos wrote on Twitter on Sept. 27, the day Mr. Barr was in Italy. “Italy holds the keys to the kingdom. Right government, right time.”
On Friday, former Prime Minister Matteo Renzi of Italy said he was suing Mr. Papadopoulos for slander because he told an Italian right-wing newspaper that Mr. Renzi, while in office, had taken orders from former President Barack Obama to try to derail Mr. Trump’s candidacy. “See you in court,” Mr. Renzi wrote on Facebook.
Mr. Papadopoulos served 12 days in prison last year for lying to F.B.I. agents in the Russia investigation, and investigators said his lies hindered their ability to question Mr. Mifsud. An Italian government official confirmed that Mr. Barr and Mr. Durham traveled to Rome in part to gain more information about Mr. Mifsud.
Mr. Barr opened the Justice Department review this year after he said he did not get “satisfactory” answers when he asked why law enforcement officials opened the 2016 counterintelligence investigation into the Trump campaign. He turned to Mr. Durham to review the origins of the F.B.I.’s Russia investigation and whether it was properly predicated.
Mr. Durham has a track record with delicate cases where the investigative focus is on F.B.I. agents and C.I.A. officers. In 1999, Attorney General Janet Reno asked him to investigate the F.B.I.’s handling of James “Whitey” Bulger, a notorious mobster whom agents used as an informant. He secured convictions and unraveled a corrupt network of law enforcement officials working with Mr. Bulger.
Almost a decade later, Mr. Durham was directed to investigate the destruction of C.I.A. videotapes depicting the torture of detainees in secret prisons run by the agency. During that investigation, he interviewed Gina Haspel, now the director of the C.I.A., about her role in the destruction of the tapes. The investigation was expanded to include abuses of C.I.A. detainees. It ended with no criminal charges.
His most recent assignment involved investigating James A. Baker, the widely respected former top lawyer at the F.B.I., over a suspected leak of classified information. Mr. Durham quietly used agents with the United States Postal Service in that case because the Justice Department had decided that the F.B.I. could not investigate itself, people familiar with the investigation said. Mr. Baker has denied wrongdoing and was never charged with a crime.
For his review, Mr. Durham has enlisted Nora R. Dannehy, a veteran federal prosecutor who worked with him in Connecticut and led a two-year inquiry into whether department officials under President George W. Bush broke the law in firing several United States attorneys.
Many of the F.B.I. and C.I.A. officials that Mr. Durham is expected to attempt to interview have left government, including Bill Priestap, the bureau’s top counterintelligence agent during the Russia inquiry. Mr. Priestap privately told Congress last year that there was no F.B.I. conspiracy against Mr. Trump or his campaign.
He was also asked whether he met Mr. Mifsud on an overseas trip, a suggestion the F.B.I. was secretly working with the professor. Mr. Priestap said no.
For his part, Mr. Barr does not seem to mind that his travels in aid of the Durham review create an appearance that he is trying to protect the president. During a speech on Thursday, Mr. Barr recalled a recent episode when he was asked which country he planned to visit next. “Greenland,” he joked, a reference to one of Mr. Trump’s previous controversies.
Jason Horowitz contributed reporting from Rome, and Julian E. Barnes from Washington.
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