The Russia investigation took a significant step forward when reports surfaced Wednesday that special counsel Robert Mueller is questioning witnesses about what, and how much, President Trump knew about Russia's hack of the Democratic National Committee.
While Trump is a focus of the obstruction-of-justice thread of the probe, he has not been a subject in the collusion inquiry — until now.
"Prosecutors typically move up the food chain" in these types of investigations, said one legal expert and former intelligence official.
For that reason, Mueller's latest focus means he likely has enough evidence in the collusion thread "to begin putting a story together that involves Trump."
On Wednesday, special counsel Robert Mueller's investigation into whether President Donald Trump's campaign colluded with Moscow during the 2016 election reportedly set its sights, for the first time, on 1600 Pennsylvania Avenue.
A new report from NBC News said Mueller is scrutinizing what, and how much, Trump knew about the Russian-backed campaign to hack into the Democratic National Committee in the summer of 2016, and whether Trump had any role in the radical pro-transparency group WikiLeaks' subsequent dissemination of the stolen emails.
Mueller is overseeing the FBI's investigation into Russia's interference in the 2016 election. The probe, broadly, has two main threads vis-a-vis the Trump campaign: whether members of the campaign colluded with Moscow to tilt the race in his favor, and whether Trump sought to obstruct justice when he fired FBI director James Comey last May.
Trump is the focus — indeed, the catalyst — of the obstruction inquiry. But Wednesday's report marks the first time, according to public knowledge, that prosecutors are eyeing the president as they investigate possible collusion with a hostile foreign power.
Since Comey publicly confirmed the existence of the Russia investigation last year, Trump has hammered home the assertion that neither he nor his campaign colluded with Russia.
RELATED: Key Trump officials, advisers of note in the Russia probe
Key Trump officials, advisers of note in the Russia probe
Key Trump officials, advisers of note in the Russia probe
The close friend to Donald Trump and CEO of private equity firm Colony Capital recommended that Trump bring in Paul Manafort for his presidential campaign.
R. James Woolsey
Woolsey, the former director of the Central Intelligence Agency (CIA), has cooperated with Mueller's investigation and worked with Michael Flynn and was present at a meeting where they discussed removing the controversial Turkish Muslim cleric Fetullah Gulen from US soil.
(Christopher Goodney/Bloomberg via Getty Images)
The former senior Trump campaign official and White House adviser was present and crucial during the firings of Michael Flynn and James Comey.
The former head of the Trump transition team following the 2016 election has said previously that he believes he was fired due to his opposing the hiring of Michael Flynn as national security adviser.
Former U.S. senator Jeff Sessions from Alabama joined Trump's campaign as a foreign policy adviser in February 2016. Sessions was nominated to be U.S. attorney general by President Trump and was then confirmed by the Senate. Reports then emerged that Sessions had spoken twice with Sergey Kislyak while he was senator -- a fact that he left out of his Senate hearing testimony. Instead, he said in writing that he had not communicated with any Russian officials during the campaign season. Sessions defended himself saying he had spoken with Kislyak specifically in a senate capacity.
Paul Manafort signed on as Donald Trump's campaign manager in March 2016. A longtime Republican strategist and beltway operative, Manafort had previously served as an adviser to former Ukrainian president Viktor Yanukovich -- a pro-Russia leader who was violently ousted in 2014. Manafort resigned from his campaign position in August 2016 amid questions over his lobbying history in Ukraine for an administration supportive of Russia. The former campaign manager reportedly remained in Trump's circle during the post-election transition period.
Gen. Michael Flynn was named President Trump's national security adviser in November of 2016. Flynn reportedly met and spoke with Russian Ambassador Sergey Kislyak in December, at one point discussing sanctions. Flynn originally told Vice President Pence he did not discuss sanctions -- a point the Department of Justice said made the national security adviser subject to blackmail. Flynn resigned from his position in February.
2016 election winner Donald Trump is at the center of special counsel Robert Mueller's probe into Russia's handlings.
Clovis, a former member of the Trump campaign, arrives on at the U.S. Capitol December 12, 2017 to appear before a closed meeting of the House Intelligence Committee. Clovis worked with George Papadopoulos, a former Donald Trump campaign foreign policy advisor who struck a plea deal on charges of lying to the FBI.
(Photo by Win McNamee/Getty Images)
Stone is a longtime Republican political consultant who served as a campaign adviser to Trump who continued to talk with the then-GOP candidate after stepping away from his adviser role. Stone claimed last year that he had knowledge of the planned WikiLeaks release of emails pertaining to Hillary Clinton and the Democratic National Committee. Stone recently admitted to speaking via direct message with "Guccifer 2.0" -- an online entity U.S. officials believe is tied to Russia. Stone says the correspondence was “completely innocuous.”
Page worked for Merrill Lynch as an investment banker out of their Moscow office for three years before joining Trump's campaign as a foreign policy adviser. During his time with Merrill Lynch, Page advised transactions for two major Russian entities. Page has called Washington "hypocritical" for focusing on corruption and democratization in addressing U.S. relations with Russia. While Page is someone Trump camp has seemingly tried to distance itself from, Page recently said he has made frequent visits to Trump Tower.
Before Gordon joined the Trump campaign as a national security adviser in March 2016, he served as a Pentagon spokesman from 2005 through 2009. Like others involved in Trump-Russia allegations, Gordon met with ambassador Kislyak in July at the Republican National Convention, but has since denied any wrongdoing in their conversation. He advocated for and worked to revise the RNC language on and position toward Ukraine relations, so it was more friendly toward Russia's dealings in the country.
Former Trump campaign aide Michael Caputo (L)
Caputo waves goodbye to reporters after he testified before the House Intelligence Committee during a closed-door session at the U.S. Capitol Visitors Center July 14, 2017 in Washington, DC. Caputo resigned from being a Trump campaign communications advisor after appearing to celebrate the firing of former campaign manager Corey Lewandowski. Denying any contact with Russian officials during the 2016 campaign, Caputo did live in Moscow during the 1990s, served as an adviser to former Russian President Boris Yeltsin and did pro-Putin public relations work for the Russian conglomerate Gazprom Media.
(Photo by Chip Somodevilla/Getty Images)
Stephen Miller, White House Senior Advisor for Policy
Former White House Press Secretary Sean Spicer
Donald Trump Jr.
White House Senior adviser Jared Kushner
Executive assistant to Donald Trump Rhona Graff
White House Communications Director Hope Hicks
Former Trump campaign manager Corey Lewandowski
US Vice President Mike Pence
Former White House Communications Director Anthony Scaramucci
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"It is now commonly agreed, after many months of COSTLY looking, that there was NO collusion between Russia and Trump," he tweeted last October.
Following the House Intelligence Committee's release of a controversial Republican memo purporting to show surveillance abuses by the FBI and Department of Justice when they sought to monitor a Trump campaign adviser, Trump tweeted again.
"This memo totally vindicates 'Trump' in probe," the president said. "But the Russian Witch Hunt goes on and on. Their was no Collusion and there was no Obstruction (the word now used because, after one year of looking endlessly and finding NOTHING, collusion is dead). This is an American disgrace!"
But Mueller's actions over the last few weeks appear to be sending another signal entirely.
Moving 'up the food chain'
Earlier this month, the special counsel's office indicted 13 Russian nationals and three Russian entities, charging them with conspiring to interfere in the 2016 race by mounting an elaborate social-media disinformation campaign aimed at sowing discord leading up to and after the election.
The indictment laid out a stark and highly specific picture of how the Russians carried out their scheme, which the court filing said they undertook with the specific purpose of boosting Trump and denigrating his opponent.
The document did not name any Americans as willing co-conspirators, and it did not make a judgment on whether the defendants' actions had an effect on the outcome of the election — a fact the president trumpeted.
"Russia started their anti-US campaign in 2014, long before I announced that I would run for President," Trump tweeted the day the indictments were unsealed. "The results of the election were not impacted. The Trump campaign did nothing wrong - no collusion!"
But legal experts warned at the time that the indictment could be just the first step in a broader examination of whether — and to what extent — any Americans aided the Russians' efforts.
Collusion, as Trump and his allies have repeatedly pointed out, is not in and of itself a crime. Mueller's approach to the inquiry will therefore likely be tethered to proving two key assertions: that a conspiracy to defraud the US took place by way of attempting to interfere in the election, and that Americans had knowledge of, and acted to further, that conspiracy.
Wednesday's report appears to be a public confirmation of Mueller's attention to the latter.
In particular, investigators are said to be interested in Trump's public appeal for Russia to recover then-Democratic candidate Hillary Clinton's deleted emails during a press conference on July 27, 2016.
"Russia, if you're listening, I hope you're able to find the 30,000 emails that are missing," Trump said at the time.
Mueller's new focus on Trump as it relates to Russia's DNC hack and WikiLeaks' actions "is a significant development," said Jens David Ohlin, a vice dean at Cornell Law School and an expert on criminal law.
"In these kinds of investigations, prosecutors typically move up the food chain," said Robert Deitz, who served as the former general counsel at the National Security Agency. "I suspect that Mueller, through earlier interviews or documents, finally has sufficient evidence to begin putting a story together that involves Trump."
The special counsel's scrutiny comes as Trump's lawyers are angling to sidestep a face-to-face interview between their client, who has shown a tendency to exaggerate the truth, and Mueller.
"The one specious thing about Trump's lawyers' strategy is that they're saying there's no substantive focus on Trump when it comes to the collusion inquiry, that this is all about obstruction of justice, and that Mueller already has everything he needs from other witnesses and documents for the obstruction case," said Andrew Wright, who served in the White House counsel's office under former President Barack Obama.
"Setting the obstruction case aside, what has happened over the last couple of weeks is that Mueller laid the foundation" for the collusion inquiry "by showing the crimes of the Russians, in at least some form," he said. "And now we're seeing his focus on Trump about the hacked emails. At this point, it's a matter of proving whether Americans were participants in this conspiracy to commit these crimes, and whether they aided or abetted these acts."
Filling in the dots
Trump has had no known communications with WikiLeaks, an organization the US intelligence community believes is a tool of the Russian government.
But candidate Trump demonstrated a marked affinity for the group in the past, repeatedly praising it in the month leading up to the November election.
"It's amazing how nothing is secret today when you talk about the internet," he said at a rally on October 6, 2016, adding that he loved WikiLeaks.
He also tweeted about WikiLeaks on October 11, writing, "I hope people are looking at the disgraceful behavior of Hillary Clinton as exposed by WikiLeaks. She is unfit to run."
WikiLeaks reached out to Trump's eldest son, Donald Trump Jr., via Twitter direct message on October 12, telling him it was "great" to see him and Trump "talking about our publications." It "strongly" suggested that Trump tweet out the link wlsearch.tk, claiming the site would help people search through the hacked documents.
WikiLeaks also told Trump Jr. it had just released another batch of emails belonging to Clinton campaign manager John Podesta.
RELATED: A look at Robert Mueller
Former FBI Director Robert Mueller
Former FBI Director Robert Mueller
WASHINGTON, DC - OCTOBER 28: Former FBI director Robert Mueller attends the ceremonial swearing-in of FBI Director James Comey at the FBI Headquarters October 28, 2013 in Washington, DC. Comey was officially sworn in as director of FBI on September 4 to succeed Mueller who had served as director for 12 years. (Photo by Alex Wong/Getty Images)
US President Barack Obama applauds outgoing Federal Bureau of Investigations (FBI) director Robert Mueller (L) in the Rose Garden at the White House in Washington on June 21, 2013 as he nominates Jim Comey to be the next FBI director. Comey, a deputy attorney general under George W. Bush, would replace Mueller, who is stepping down from the agency he has led since the week before the September 11, 2001 attacks. AFP PHOTO/Nicholas KAMM (Photo credit should read NICHOLAS KAMM/AFP/Getty Images)
Outgoing FBI Director Robert Mueller applauds key staff members during a farewell ceremony held for him at the Justice Department in Washington, August 1, 2013. On Monday the U.S. Senate confirmed former Deputy Attorney General James Comey to replace Mueller, who has led the bureau since shortly before the September 11, 2001, attacks on the United States. REUTERS/Jonathan Ernst (UNITED STATES - Tags: POLITICS CRIME LAW HEADSHOT)
391489 03: U.S. President George W. Bush speaks during a conference as he stands with Justice Department veteran Robert Mueller, left, who he has nominated to head the FBI, and Attorney General John Ashcroft July 5, 2001 the Rose Garden at the White House in Washington, DC. (Photo by Mark Wilson/Getty Images)
Outgoing FBI Director Robert Mueller stands for the national anthem during a farewell ceremony for him at the Justice Department in Washington, August 1, 2013. On Monday the U.S. Senate confirmed former Deputy Attorney General James Comey to replace Mueller, who has led the bureau since shortly before the September 11, 2001, attacks on the United States. REUTERS/Jonathan Ernst (UNITED STATES - Tags: POLITICS CRIME LAW)
Outgoing FBI Director Robert Mueller (L) reacts to a standing ovation from the audience, Deputy U.S. Attorney General James Cole (C) and U.S. Attorney General Eric Holder (R) during Mueller's farewell ceremony at the Justice Department in Washington, August 1, 2013. On Monday the U.S. Senate confirmed former Deputy Attorney General James Comey to replace Mueller, who has led the bureau since shortly before the September 11, 2001, attacks on the United States. REUTERS/Jonathan Ernst (UNITED STATES - Tags: POLITICS CRIME LAW)
Outgoing FBI Director Robert Mueller gestures during his remarks at a farewell ceremony held for him at the Justice Department in Washington, August 1, 2013. On Monday the U.S. Senate confirmed former Deputy Attorney General James Comey to replace Mueller, who has led the bureau since shortly before the September 11, 2001, attacks on the United States. REUTERS/Jonathan Ernst (UNITED STATES - Tags: POLITICS CRIME LAW)
FILE PHOTO -- U.S. Attorney General John Ashcroft (R) and FBI Director Robert Mueller speak about possible terrorist threats against the United States, in Washington, May 26, 2004. REUTERS/Kevin Lamarque/File Photo
Outgoing FBI Director Robert Mueller reacts to applause from the audience during his farewell ceremony at the Justice Department in Washington, August 1, 2013. On Monday the U.S. Senate confirmed former Deputy Attorney General James Comey to replace Mueller, who has led the bureau since shortly before the September 11, 2001, attacks on the United States. REUTERS/Jonathan Ernst (UNITED STATES - Tags: POLITICS CRIME LAW)
UNITED STATES - JUNE 19: Chairman Pat Leahy, D-Vt., right, and FBI Director Robert Mueller make their way to a Senate Judiciary Committee hearing in Dirksen Building on oversight of the FBI. (Photo By Tom Williams/CQ Roll Call)
Outgoing FBI Director Robert Mueller (C) delivers remarks at a farewell ceremony for him at the Justice Department in Washington, August 1, 2013. On Monday the U.S. Senate confirmed former Deputy Attorney General James Comey to replace Mueller, who has led the bureau since shortly before the September 11, 2001, attacks on the United States. Also onstage with Mueller are Deputy U.S. Attorney General James Cole (FROM L), U.S. Attorney General Eric Holder, former CIA Director George Tenet and TSA Administrator John Pistole. REUTERS/Jonathan Ernst (UNITED STATES - Tags: POLITICS CRIME LAW)
WASHINGTON, DC - MAY 15: (L-R) Rep. Steny Hoyer (D-MD), U.S. Attorney General Eric Holder, Homeland Security Secretary Janet Napolitano, FBI Director Robert Mueller and Immigration and Customs Enforcement Director John Morton attend the National Peace Officers' Memorial Service at the U.S. Capitol May 15, 2013 in Washington, DC. Holder and other members of the Obama administration are being criticized over reports of the Internal Revenue Services' scrutiny of conservative organization's tax exemption requests and the subpoena of two months worth of Associated Press journalists' phone records. (Photo by Chip Somodevilla/Getty Images)
FBI Director Robert Mueller testifies before the House Judiciary Committee hearing on Federal Bureau of Investigation oversight on Capitol Hill in Washington June 13, 2013. Mueller said on Thursday that the U.S. government is doing everything it can to hold confessed leaker Edward Snowden accountable for splashing surveillance secrets across the pages of newspapers worldwide. REUTERS/Yuri Gripas (UNITED STATES - Tags: POLITICS CRIME LAW)
Ukrainian President Viktor Yanukovych (L) welcomes FBI Director Robert Mueller during their meeting in Kiev June 5, 2013. REUTERS/Efrem Lukatsky/Pool (UKRAINE - Tags: POLITICS)
FBI Director Robert Mueller (L) arrives for the Obama presidential inauguration on the West Front of the U.S. Capitol January 21, 2013 in Washington. President Barack Obama was re-elected for a second term as President of the United States. Woman at right is unidentified. REUTERS/Win McNamee-POOL (UNITED STATES)
WASHINGTON, : FBI Director Robert Mueller answers questions before Congress 17 October 2002 on Capitol Hill in Washington. Mueller was testifying before the House and Senate Select Intelligence committees' final open hearing investigating events leading up to the September 11, 2001. AFP Photos/Stephen JAFFE (Photo credit should read STEPHEN JAFFE/AFP/Getty Images)
(L-R) CIA Director Leon Panetta, Director of National Intelligence James Clapper and FBI Director Robert Mueller testify at a Senate Intelligence Committee hearing on Capitol Hill in Washington, February 16, 2011. REUTERS/Jason Reed (UNITED STATES - Tags: POLITICS)
399994 02: Federal Bureau of Investigation (FBI) Director Robert Mueller visits the American military compound at Kandahar Airport January 23, 2002 in Kandahar, Afghanistan. Mueller had lunch with FBI officials and Haji Gulali, commander of the Kandahar region. (Photo by Mario Tama/Getty Images)
Federal Bureau of Investigation (FBI) Director Robert Mueller (L) stand during the National Anthem alongside Attorney General Eric Holder (R) and Deputy Attorney General James Cole (C) during a farewell ceremony in Mueller's honor at the Department of Justice on August 1, 2013. Mueller is retiring from the FBI after 12-years as Director. AFP PHOTO / Saul LOEB (Photo credit should read SAUL LOEB/AFP/Getty Images)
399994 01: Federal Bureau of Investigation (FBI) Director Robert Mueller greets American forces on the American military compound at Kandahar Airport January 23, 2002 in Kandahar, Afghanistan. Mueller had lunch with FBI officials and Haji Gulali, commander of the Kandahar region. (Photo by Mario Tama/Getty Images)
UNITED STATES - JUNE 19: FBI Director Robert Mueller, center, talks with Chairman Pat Leahy, D-Vt., right, and Sen. Orrin Hatch, R-Utah, talk before a Senate Judiciary Committee hearing in Dirksen Building on oversight of the FBI. (Photo By Tom Williams/CQ Roll Call)
UNITED STATES - JUNE 06: OVERSIGHT HEARING ON COUNTERTERRORISM--Sen. Richard J. Durbin, D-Ill., Senate Judiciary Chairman Patrick J. Leahy, D-Vt., FBI Director Robert S. Mueller III, and Sen. Charles E. Grassley, R-Iowa, before the hearing. (Photo by Scott J. Ferrell/Congressional Quarterly/Getty Images)
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An hour later, Trump tweeted: "Very little pick-up by the dishonest media of incredible information provided by WikiLeaks. So dishonest! Rigged system!"
Longtime former federal prosecutor Jeffrey Cramer, who is now the managing director at Berkeley Research Group, emphasized that while Trump's statements on the campaign trail are important to Mueller's case, they are not, in and of themselves, illegal.
"It could be one data point as the investigators delve into whether there was any connection between Trump or his campaign and WikiLeaks of the Russians," Cramer said. "Relevant facts to make a case would include whether anyone connected with the campaign assisted the release in any way."
Trump Jr. was in touch with WikiLeaks on multiple occasions between September 2016 and July 2017. Based on public reporting, the bulk of Trump Jr.'s interactions with WikiLeaks via Twitter direct message took place in October 2016.
In addition to scrutinizing Trump, Mueller's team is also reportedly looking into Republican strategist Roger Stone's contacts with WikiLeaks, Assange, and the Russia-linked hacker Guccifer 2.0, whom US intelligence agencies have characterized as a front for Russian military intelligence.
Stone worked as an informal adviser to the Trump campaign until August 2015. He was in direct contact with WikiLeaks in mid-October 2016 and they had a brief back-and-forth, according to The Atlantic.
On the morning after Trump won the 2016 election, WikiLeaks reportedly messaged Stone, saying, "Happy? We are now more free to communicate."
Stone's relationship with Trump has also sparked prosecutors' interest.
One witness interviewed by Mueller's team told NBC News that investigators asked about what Stone's interactions with Trump were like once he ended his tenure as a Trump campaign adviser in August 2015.
"How often did they talk? Who really fired him? Was he really fired?" the witness said, describing the questions they were asked.
Deitz said Mueller "will continue to fill in the dots until he has enough to seek an interview with (or indictment of) Trump."