Mueller's prosecutors described in the report released on Thursday how Joseph Mifsud tried to broker meetings between Papadopoulos and Russians. It also reported that Mifsud had interactions with a former Russian troll factory employee and contacts linked to the Russian military unit accused of hacking into Clinton's campaign and the Democratic Party's emails.
On Thursday evening, Papadopoulos' wife Simona Papadopoulos tweeted out an Italian news article that reported that the formerly London-based professor, who apparently disappeared from public view early last year, had been "hiding in Rome" for a seven-month stretch in an apartment paid for by a university where Mifsud had taught a course in political science.
Mifsud's profile on the school's website was taken down in January 2018 and Mifsud hasn't been seen at the university in some time, according to a BuzzFeed News report.
There was even an unconfirmed report this past year – which surfaced in a Democratic Party lawsuit -- that Mifsud had may have died. At the time, a man identifying himself as the professor’s lawyer told ABC News and others that the report was “nonsense.”
Another mystery man who has resurfaced with the conclusion of the Mueller probe is Belarusan-American businessman Sergei Millian. He has been identified by news reports, and later corroborated by sources interviewed by ABC News, as an unwitting source of the some information contained in infamous dossier compiled by former British spy Christopher Steele for Washington-based research firm Fusion GPS.
Millian has long denied this claim.
A naturalized U.S. citizen, Millian became active in supporting the Trump campaign while still promoting his nonprofit group, called the Russian American Chamber of Commerce. But as figures central to the investigation, including Papadopoulos and Trump's former longtime personal attorney Michael Cohen, began describing interactions with him, he largely disappeared from public sight.
Congressional investigators last year tried for months to serve him a subpoena to testify, but their process servers could not find him. Several people who knew Millian told ABC News he had vanished.
A July 2016 interview with ABC News is his only appearance on U.S. television.
The Mueller report indicated that Millian in August 2016 sent a Facebook message to Papadopoulos offering to share with him "a disruptive technology that might be instrumental in your political work for the campaign."
Papadopoulos claimed to have no recollection of the matter, according to the report.
Mueller's prosecutors went on to say that their team was "not fully able to explore the contact" Millian had with various figures in Trump’s orbit because he remained out of their grasp.
Nonetheless, Millian on Thursday tweeted that he is "feeling totally exonerated by the recent report," and urged public officials to "find the truth" about Steele and Fusion GPS founder Glenn Simpson.
Millian has tweeted several Mueller probe-related comments over the weekend, celebrating "truth coming to light" and calling his experience over the last three years an "unprecedented smear campaign."
Not all of the figures described in the Mueller report have maintained quite such a low profile.
Washington-based Russian expert Dimitri Simes, who appears to have had influence on Jared Kushner's foreign policy platform, based on the Mueller report, has made occasional appearances during the past two years – though more frequently in Russian media outlets.
The Mueller report says that during the 2016 campaign, Simes, who heads a think tank called the Center for the National Interest, prepared a "Russia Policy Memo" for Kushner.
In it, Simes described "a well-documented story of highly-questionable connections between Bill Clinton" and the Russian government. The memo was forwarded to senior Trump campaign officials, including then-Trump campaign Paul Manafort and deputy chairman Rick Gates.
Mueller's prosecutors described Simes in the report as personally having "many contacts with current and former Russian government officials," and cited his think tank's own description of itself as having "unparalleled access to Russian officials and politicians among Washington think tanks." According to the report, the center grew out of an organization founded by former President Richard Nixon.
According to the report, however, Simes had also previously advised Kushner that it was "bad optics" for the campaign to develop "hidden Russian contacts," and told Kushner to not "highlight Russia as an issue" and to handle any Russian contacts with care, according to the report.
The report also revealed that after the 2016 election, Simes was contacted by an associate working for Petr Aven, the head of Russia's biggest bank -- Alfa Bank -- to establish direct communication between the Trump administration and the Russian government. The report said he declined, saying he did not want to be seen as an intermediary between Trump world and Kremlin.
Rinat Akhmetshin, a well-known Russian-focused lobbyist in Washington, D.C., attended the infamous June 2016 meeting at the Trump Tower. He said recently he was "happy and relieved" the investigation was winding down after nearly two years.
"It hit me hard financially and led to baseless personal attacks," Rinat Akhmetshin told ABC News in a statement, saying that the process took a "heavy toll on me and my family."
"As a result," Akhmetshin added, "my ability to earn a living has been impaired, my professional standing has been undermined and my personal relationships have suffered."