President Donald Trump reportedly asked Deputy Attorney General Rod Rosenstein whether he was "on my team" during a meeting in December.

The interaction is the third known instance during which Trump asked a top law-enforcement official where their loyalties lie.

Rosenstein is increasingly emerging as a crucial witness in the Russia investigation, and experts said the reported interaction is an important new piece of evidence for special counsel Robert Mueller as he examines whether Trump sought to obstruct justice in the Russia probe.

The special counsel, Robert Mueller, may have been handed another key piece of the puzzle as he investigates Russia's interference in the 2016 US election and whether President Donald Trump sought to obstruct justice.

CNN reported Wednesday that Trump asked Rod Rosenstein, the deputy attorney general, if he was "on my team" during a meeting at the White House in December.

Rosenstein reportedly replied, "Of course, we're all on your team, Mr. President."

Trump also asked Rosenstein about how the Russia investigation was coming along, which Rosenstein declined to comment on, according to the report. Their meeting came as Rosenstein was preparing to testify before the House Judiciary Committee.

12 PHOTOS Rod Rosenstein through the years See Gallery Rod Rosenstein through the years Rod Rosenstein, nominee to be Deputy Attorney General, testifies before the Senate Judiciary Committee on Capitol Hill in Washington March 7, 2017. REUTERS/Aaron P. Bernstein UNITED STATES - OCTOBER 10: U.S. Attorney Rod Rosenstein speaks during a news conference in Washington D.C. Tuesday, October 10, 2006. Rosenstein and Deputy U.S. Attorney General Paul McNulty announced the formation of a National Procurement Fraud Task Force, an effort aimed at the detection, prevention and prosecution of procurement fraud associated with increased contracting activity for national security programs. (Photo by Carol T. Powers/Bloomberg via Getty Images) UNITED STATES - OCTOBER 10: Deputy U.S. Attorney General Paul McNulty, center, speaks during a news conference with Alice Fisher, head of the criminal division of the U.S. Department of Justice, left, and U.S. Attorney Rod Rosenstein, during a news conference in Washington D.C. Tuesday, October 10, 2006. McNulty announced the formation of a National Procurement Fraud Task Force, an effort aimed at the detection, prevention and prosecution of procurement fraud associated with increased contracting activity for national security programs. (Photo by Carol T. Powers/Bloomberg via Getty Images) SLUG: me/hornsby DATE: August 22, 2006 CREDIT: Ricky Carioti / TWP. United States Federal Courthouse in Greenbelt, Md. Federal prosecutors announce the indictment of former Prince George's County school superintendent Andre Hornsby. United States Attorney Rod J. Rosenstein, center, flanked by Francis Turner, left, of the United States Department of the Treasury and Assistant United States Attorney Michael Pauze announce the 16-count indictment of former Prince George's County Schools Superintendent Andre Hornsby during a press conference at federal court in Greenbelt on Tuesday. (Photo by Ricky Carioti/The Washington Post/Getty Images) U.S. Attorney Rod Rosenstein talks about the sentencing of Thomas Bromwell Sr. and Mary Patricia Bromwell following their appearance in federal court in Baltimore, Maryland, Friday, November 16, 2007. (Photo by Kim Hairston/Baltimore Sun/MCT via Getty Images) GREENBELT, MD JUNE 30:United States Attorney for the District of Maryland Rod J. Rosenstein talked with reporters after the Guilty plea of Prince Georges County Councilwoman Leslie Johnson the U.S. District Court on June 30, 2011 in Greenbelt, MD. To Rosenstein's left is Acting Special Agent in Charge Jeannine A. Hammett of the Internal Revenue Service and to his right is Special Agent in Charge Richard A. McFeely of the Federal Bureau of Investigation. (Photo by Mark Gail/The Washington Post via Getty Images) BALTIMORE, MD - OCTOBER 24: Maryland U.S. Attorney Rod J. Rosenstein, on Friday, October 24, 2014 in Baltimore, Maryland. Rosenstein said Carl Lackl was scheduled to be a witness to the Larry Haynes murder but was killed when Patrick Byers plotted his murder from his jail cell. (Photo by Michel du Cille/The Washington Post via Getty Images) Rod Rosenstein, deputy attorney general nominee for U.S. President Donald Trump, listens during a Senate Judiciary Committee confirmation hearing in Washington, D.C., U.S., on Tuesday, March 7, 2017. The confirmation hearing for Rosenstein began with Republicans and Democrats squaring off over who should lead probes into Russian interference in the 2016 presidential election and potential contacts between Moscow and Trumps campaign team. Photographer: Andrew Harrer/Bloomberg via Getty Images Rod Rosenstein, deputy attorney general nominee for U.S. President Donald Trump, swears in to a Senate Judiciary Committee confirmation hearing in Washington, D.C., U.S., on Tuesday, March 7, 2017. The confirmation hearing for Rosenstein began with Republicans and Democrats squaring off over who should lead probes into Russian interference in the 2016 presidential election and potential contacts between Moscow and Trumps campaign team. Photographer: Andrew Harrer/Bloomberg via Getty Images Rod Rosenstein, deputy attorney general nominee for U.S. President Donald Trump, sits during a Senate Judiciary Committee confirmation hearing in Washington, D.C., U.S., on Tuesday, March 7, 2017. The confirmation hearing for Rosenstein began with Republicans and Democrats squaring off over who should lead probes into Russian interference in the 2016 presidential election and potential contacts between Moscow and Trumps campaign team. Photographer: Andrew Harrer/Bloomberg via Getty Images WASHINGTON, DC - MARCH 07: Deputy U.S. Attorney General nominee Rod Rosenstein arrives before the Senate Judiciary Committee for testimony March 7, 2017 in Washington, DC. During the hearing, Democratic senators pressed Rosenstein to appoint a special prosecutor in an ongoing federal inquiry into Russian influence in the U.S. presidential election. (Photo by Win McNamee/Getty Images) Rod Rosenstein, nominee to be Deputy Attorney General, arrives to testify before the Senate Judiciary Committee on Capitol Hill in Washington March 7, 2017. REUTERS/Aaron P. Bernstein Up Next See Gallery Discover More Like This HIDE CAPTION SHOW CAPTION of SEE ALL BACK TO SLIDE

Rosenstein reportedly went to the White House at the time to request Trump's help in pushing back against document requests from embattled House Intelligence Committee chairman Devin Nunes, who has been conducting his own investigation into alleged bias at the Department of Justice and FBI.

Instead, Trump was said to have focused on Rosenstein's upcoming House Judiciary Committee testimony, the report said.

The hearing, which took place on December 13 last year, was scheduled as part of the committee's normal oversight functions. But Republicans spent most of their time grilling Rosenstein about whether they perceived that bias by some former agents who worked on Mueller's team had tainted the entire Russia investigation.

A consistent pattern

The interaction between Trump and Rosenstein marks the third known instance during which the president asked a top law-enforcement official where their loyalties lie.

The first occurred last year, shortly after Trump's inauguration. Former FBI director James Comey said, in his prepared remarks for the Senate Intelligence Committee in June 2017, that Trump asked him for his loyalty during a private dinner months earlier, in January.

Comey said he declined to pledge his loyalty to Trump, adding that the interaction "concerned" him "greatly" because he believed Trump was "trying to create some sort of patronage relationship," in which Comey would keep his job as FBI director as long as he remained loyal to the president.

The next month, Trump asked Comey to let go of the FBI's investigation into former national security adviser Michael Flynn, who had resigned one day before. Three months later, after Comey refused to drop the Flynn probe, Trump fired him.

The White House first said Comey had been fired because of how he handled the investigation into former Secretary of State Hillary Clinton's use of a private email server, but Trump later told NBC's Lester Holt that "this Russia thing" had been a factor in his decision.

9 PHOTOS Former FBI Deputy Director Andrew McCabe See Gallery Former FBI Deputy Director Andrew McCabe Newly installed acting FBI Director Andrew McCabe testifies before the Senate Intelligence Committee on Capitol Hill in Washington, DC, May 11, 2017. / AFP PHOTO / JIM WATSON (Photo credit should read JIM WATSON/AFP/Getty Images) UNITED STATES - MAY 11: Acting FBI Director Andrew McCabe prepares to testify during the Senate (Select) Intelligence Committee hearing on 'World Wide Threats' on Thursday, May 11, 2017. (Photo By Bill Clark/CQ Roll Call) UNITED STATES - MAY 11: Acting FBI Director Andrew McCabe prepares to testify during the Senate (Select) Intelligence Committee hearing on 'World Wide Threats' on Thursday, May 11, 2017. (Photo By Bill Clark/CQ Roll Call) UNITED STATES - MAY 11: From left, Acting FBI Director Andrew McCabe, CIA Director Mike Pompeo, and Director of National Intelligence Daniel Coats, appear during a Senate (Select) Intelligence Committee hearing in Hart Building titled 'World Wide Threats' on May 11, 2017. (Photo By Tom Williams/CQ Roll Call) WASHINGTON, USA - MAY 11: Andrew McCabe, Acting Director of the FBI after President Trump fired James Comey, speaks during a Senate Select Committee on Intelligence hearing on Worldwide Threats in Washington, USA on May 11, 2017. (Photo by Samuel Corum/Anadolu Agency/Getty Images) WASHINGTON, USA - MAY 11: A binder containing classified material marked Secret sits on the witness table in front of Andrew McCabe, Acting Director of the FBI after President Trump fired James Comey, before a Senate Select Committee on Intelligence hearing on Worldwide Threats in Washington, United States on May 11, 2017. (Photo by Samuel Corum/Anadolu Agency/Getty Images) WASHINGTON, DC - DECEMBER 21: Federal Bureau of Investigation Deputy Director Andrew McCabe arrives for a meeting with members of the Oversight and Government Reform and Judiciary committees in the Rayburn House Office Building December 21, 2017 in Washington, DC. McCabe testified before the House Intelligence Committee for ten hours on Tuesday. (Photo by Chip Somodevilla/Getty Images) US Attorney General Jeff Sessions (C) speaks with Health and Human Services Secretary Tom Price (L) and Acting Director of the Federal Bureau of Investigation (FBI) Andrew McCabe (R) during a press conference at the US Department of Justice in Washington, DC, on July 13, 2017. / AFP PHOTO / JIM WATSON (Photo credit should read JIM WATSON/AFP/Getty Images) Acting FBI Director Andrew McCabe testifies before the House Commerce, Justice, Science, and Related Agencies Subcommittee on Capitol Hill in Washington, U.S., June 21, 2017. REUTERS/Aaron P. Bernstein Up Next See Gallery Discover More Like This HIDE CAPTION SHOW CAPTION of SEE ALL BACK TO SLIDE

The second instance during which Trump questioned a top official's loyalty took place shortly after Comey's firing. The Washington Post reported last week that following Comey's ouster, Trump met with then-acting FBI director Andrew McCabe and asked him who he voted for during the 2016 presidential election. McCabe, who reportedly said he did not vote in the election that year, found the question to be "disturbing," one official told The Post.

Though the White House tapped McCabe to be acting FBI director, Trump quickly soured on him as the Russia probe picked up steam last year. McCabe was finally forced out of the bureau earlier this week following months of sustained attacks on his credibility from Trump and his loyalists.

FBI director Christopher Wray indicated that McCabe's removal was the result of a Justice Department investigation into his handling of the Clinton email investigation. But the deputy director's defenders questioned the timing of the move, given that it came amid Trump's criticisms, as well as Attorney General Jeff Sessions' reported request that Wray replace McCabe at the bureau.

McCabe is one of three officials Comey apprised of his conversations with Trump, which are now being scrutinized by Mueller and congressional investigators. The other two, former FBI general counsel James Baker and Comey's former chief of staff James Rybicki, were replaced or reassigned within the bureau.

Comey's firing and Trump's subsequent actions make up the basis of Mueller's inquiry into whether he sought to obstruct justice in the Russia investigation. Obstruction of justice is broadly defined: it involves any conduct in which a person willfully interferes with the administration of justice.

That means influencing, obstructing, or impeding any kind of proceeding before a federal agency, department, court, or Congress, according to CNN legal analyst Danny Cevallos.

In order to prove obstruction of justice, prosecutors must establish that the defendant had "corrupt intent" when they made a particular decision, and experts said the Trump-Rosenstein meeting adds another piece of evidence to a growing arsenal in Mueller's hands.

Former federal prosecutor Jeffrey Cramer said Wednesday that the report "seems consistent with numerous other interactions [Trump] has had with senior law enforcement."

While asking for an update on the Russia investigation is not in itself relevant to criminal conduct, he said, "asking whether [Rosenstein] is on his team and connecting it to the Russia investigation by asking for a status update could, however, be relevant."

While obstruction cases like this rarely have a singular watershed moment that proves a crime, "the Rosenstein interaction is one more piece of the puzzle," Cramer added.

11 PHOTOS People reportedly interviewed in Robert Mueller's Russia probe See Gallery People reportedly interviewed in Robert Mueller's Russia probe U.S. Attorney General Jeff Sessions (Photo by Chip Somodevilla/Getty Images) Former FBI Director James Comey (REUTERS/Jonathan Ernst) Former White House Chief of Staff Reince Priebus (REUTERS/Joshua Roberts) Former White House press secretary Sean Spicer (REUTERS/Kevin Lamarque) White House Director of Strategic Communications Hope Hicks (Photo by Jabin Botsford/The Washington Post via Getty Images) Trump advisor Stephen Miller (REUTERS/Kevin Lamarque) President Trump's son-in-law and senior advisor Jared Kushner (bBRENDAN SMIALOWSKI/AFP/Getty Images) Don McGahn, general counsel for the Trump transition team (Photo by Drew Angerer/Getty Images) Christopher Steele, the former MI6 agent who compiled the reported Trump dossier (Photo by Victoria Jones/PA Images via Getty Images) Sam Clovis, a former member of the Trump campaign (Photo by Win McNamee/Getty Images) CIA Director Mike Pompeo Up Next See Gallery Discover More Like This HIDE CAPTION SHOW CAPTION of SEE ALL BACK TO SLIDE

Rosenstein emerges as a critical witness in the Russia probe

Rosenstein's reported interaction with the president indicates that he is becoming an increasingly crucial witness for Mueller and congressional investigators.

The December conversation represents the "same pattern of behavior that Trump engaged in with Comey," former federal prosecutor Renato Mariotti wrote on Twitter. "At this point, there can be little question that Rosenstein will be a witness for Mueller. It's hard to see how he can continue to oversee the Mueller investigation."

The possibility of Rosenstein's recusal is not new. The deputy attorney general attracted scrutiny last year when Trump said he had authored a memo that played a pivotal role in Comey's firing. Questions were raised at the time about whether Rosenstein should recuse himself from the Russia probe, but Cramer said Wednesday that the latest revelations indicated that they may soon reach fever pitch.

Mariotti largely agreed, saying Rosenstein is now a potentially critical witness for two key reasons:

The December meeting "shows that Trump engaged in the same behavior even after he knew he was under investigation for obstruction of justice and viewed analysis showing that his actions towards Comey were problematic," which represents strong evidence of his intent and mindset. If the Trump-Rosenstein interaction did take place, it would likely corroborate Comey's testimony and therefore undercut Trump's claims that Comey misinterpreted his statements about loyalty and the Flynn investigation.

If Rosenstein recuses himself, the associate attorney general, Rachel Brand, would be next in line to oversee Mueller's investigation. Trump could seek to appoint a new deputy attorney general, Cramer said, but the confirmation would likely be difficult under current circumstances.

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