They met in Moscow.

Joseph Mifsud saw her for the first time at the Bolshoi Theater. He struck up a conversation, offered to take her picture, and asked her out to dinner. Mifsud, an obscure, middle-aged academic originally from Malta, soon began dating the young Ukrainian woman. He liked to impress her by talking about his ties to important Russian officials. He told her he was friends with Sergey Lavrov, Russia’s foreign minister.

Soon he was traveling to Ukraine to visit her. He met her family and eventually asked her to marry him. They got engaged; she got pregnant.

And then, last fall, he vanished.

Anna, Joseph Mifsud’s fiancée, whose story — without her last name — was recently told in a well-crafted BuzzFeed News piece, stands at the center of one of the biggest mysteries in Special Counsel Robert Mueller’s investigation into possible collusion between Russia and Donald Trump: Where is Joseph Mifsud?

On October 30, 2017, Mueller unveiled the first charges in his Trump-Russia probe. That day, his office announced indictments against former Trump campaign manager Paul Manafort and his business associate Rick Gates, as well as a guilty plea from George Papadopoulos, a former foreign policy adviser to the Trump campaign. Papadopoulos had pleaded guilty to making false statements to the FBI in a deal approved secretly three weeks earlier, so it was clear that Papadopoulos was already cooperating with Mueller’s inquiry. The ambitious, young Greek-American Trump adviser was presumably telling Mueller everything he knew about connections between the Trump campaign and Russia.

George Papadopoulos.

In the court documents released in Papadopoulos’s case, one character jumps off the page: a mysterious and unnamed “overseas professor” who had served as an intermediary between Papadopoulos and the Russians. The documents reveal that during a meeting in April 2016, the professor had told Papadopoulos that the Russians had “dirt” on Democratic presidential candidate Hillary Clinton in the form of “thousands of emails.” That was well before the hacking and theft of Clinton-related emails was publicly known.

The court documents also show that Papadopoulos had good reason to believe the professor; he knew that Mifsud had close ties to senior Russian officials.

When Papadopoulos first talked to FBI agents, he tried to downplay Mifsud’s significance. He “told the investigating agents that the professor was ‘a nothing’ and ‘just a guy talk[ing] up connections or something.’ In truth and in fact, however, defendant PAPADOPOULOS understood that the professor had substantial connections to Russian government officials (and had met with some of those officials in Moscow immediately prior to telling defendant PAPADOPOULOS about the ‘thousands of emails’) and, over a period of months, defendant PAPADOPOULOS repeatedly sought to use the professor’s Russian connections in an effort to arrange a meeting between the [Trump] Campaign and Russian government officials.”

The Papadopoulos-Mifsud connection is now Exhibit A in the argument that Trump or those close to him colluded with Moscow to gain the White House.

When Papadopoulos’s guilty plea was made public, the press quickly identified the professor as Joseph Mifsud.

The Papadopoulos-Mifsud connection is highly suggestive of a direct link between the Trump presidential campaign and the Russian government. The relationship between the young campaign adviser and the academic with shadowy ties to Moscow reveals the existence of a secret channel through which the Russian government was able to communicate with the Trump campaign as it stole Democratic emails and weaponized them to help Trump win the presidency. The Papadopoulos-Mifsud connection is now Exhibit A in the argument that Trump or those close to him colluded with Moscow to gain the White House.

Roger Stone, a longtime political adviser and friend to President Donald Trump, arrives to speak and sign copies of his book “The Making of the President 2016” at the Boca Raton Marriott on March 21, 2017, in Florida.

When I began this series of columns about Trump and Russia for The Intercept, I believed that evidence of collusion between the Trump campaign and Moscow was thin. Collusion, I thought, was the weak link in the middle of the larger Trump-Russia narrative.

At that time, I believed there was much stronger evidence that the Russians had intervened in the 2016 election to help Trump win through a cyberoffensive targeting the American political system. I also thought there was powerful evidence that Trump and his aides had engaged in efforts to obstruct justice and impede Mueller’s investigation. Further, I suspected that congressional Republicans were engaged in a similar conspiracy to obstruct justice in Mueller’s inquiry. I was much less convinced that there was compelling evidence to show that Trump or those around him had actually conspired with the Russians to win the presidency. Like many others, I was willing to believe that Trump and his aides were too haplessly disorganized and incompetent to have coordinated with the Russians.

But as I’ve dug deeper into the evidence made public so far, I have become convinced that the case for collusion is much stronger than I thought. There are still plenty of unanswered questions, but that case is getting more persuasive as new facts come to light. (On Monday, the FBI raided the office and hotel room of one of Trump’s lawyers, although it’s unclear whether that will yield evidence of collusion.)

In addition to the channel between Papadopoulos and Mifsud, a series of other links also point toward ties between the Trump campaign and Russia. Some of these connections have been known for a long time, while details of others are still emerging.

One prominent example, of course, is the case of the inflammable Roger Stone, a longtime Trump political adviser and Manafort’s onetime lobbying partner. Stone, best known as a loud, pro-Trump cable news pundit and a political dirty trickster dating back to the Nixon era, has acknowledged that in 2016, he was in contact with Guccifer 2.0, the hacker who claimed to have turned over stolen Democratic National Committee emails to WikiLeaks during the presidential campaign. WikiLeaks then published the emails, sometimes releasing them at critical moments when they seemed designed to inflict maximum damage on Clinton’s campaign.

Guccifer 2.0 is now widely believed to be a front name used by Russian intelligence. The Daily Beast recently reported that Guccifer 2.0 has been identified by U.S. investigators as an officer in the GRU, Russia’s military intelligence arm.

Last year, Stone told CNN that his brief communications with Guccifer 2.0 were harmless, and that he had only messaged him in August 2016, after Guccifer’s role was publicly known. Stone has denied any involvement in collusion with Russia. In his testimony before the House Intelligence Committee in September, Stone said that “those who believe that there was collusion between the Trump camp and the Russian state, now say Stone ‘MUST HAVE’ been involved, but that is not based on one shred of evidence.”

In February, The Atlantic reported on private Twitter messages between Stone and WikiLeaks in October 2016. The magazine added that WikiLeaks “sought to keep its channel to Stone open after Trump won the election.”

Former Trump campaign aide Sam Nunberg leaves the U.S. District Courthouse on March 9, 2018, in Washington, D.C.

Despite Stone’s defiant denials of any role in collusion, Mueller is still investigating Stone’s contacts with Guccifer 2.0, WikiLeaks, and Julian Assange. Mueller’s continued interest in these connections became clear in March, when Ted Malloch, an American academic with ties to the Trump campaign and British right-wing politicians, was stopped as he entered the United States and subpoenaed to testify before Mueller’s inquiry. Malloch, who is based in London, told The Guardian that FBI agents asked about his relationship with Stone and whether he had ever visited the Ecuadorian embassy in London, where Assange, founder of WikiLeaks, has sought political asylum. The notion of a meeting between Stone and Assange was bolstered by Sam Nunberg, the former Trump adviser who caused a brief, comic furor last month when he camped out on cable news ranting about his unwillingness to cooperate with Mueller. Nunberg, who is now working with the special counsel, apparently gave Mueller a copy of an email from Stone dated August 4, 2016, in which Stone said: “I dined with Julian Assange last night,” the Wall Street Journal reported.

Stone has said the email was “a joke,” but the same day he sent that email, he had appeared on the radio show InfoWars claiming that he had dined with Assange and predicting “devastating” disclosures about Clinton, according to CNN.

The president’s son Donald Trump Jr. was also in contact with WikiLeaks during the 2016 campaign and afterward, also through private messages on Twitter. Last November, The Atlantic reported on the details of those messages, showing that WikiLeaks first contacted Trump Jr. in September 2016, and the president’s son had readily responded. In October 2016, according to The Atlantic, WikiLeaks sent Trump Jr. a message: “Hey Donald, great to see you and your dad talking about our publications. Strongly suggest your dad tweets this link if he mentions us. There’s many great stories the press are missing and we’re sure some of your follows will find it. Btw we just released Podesta Emails Part 4.”

Two days later, Trump Jr. tweeted the link.

Then-Sen. Jeff Sessions, R-Ala., second from left, speaks with a guest while Russian Ambassador to the U.S. Sergey Kislyak waits to hear Republican U.S. presidential hopeful Donald Trump speak at the Mayflower Hotel on April 27, 2016, in Washington, D.C.

Ahandful of communications might be easily explained away. The problem for Trump is that evidence of suspicious contacts with the Russians or possible intermediaries keeps mounting, and the details of those contacts make it increasingly difficult to dismiss them all as merely coincidental.

Among the contacts that are now known to have occurred are many that raise questions about their purpose, questions that Trump and his aides and allies have failed to adequately answer.

In April 2016, for example, Trump gave a foreign policy speech at the Mayflower Hotel in Washington. At the same event, then-Sen. Jeff Sessions, a Trump supporter, met with the Russian ambassador to the United States, Sergey Kislyak. Kislyak told Moscow that he had talked about campaign-related issues with Sessions, the Washington Post later reported. Sessions met with Kislyak again in September 2016 at his Senate office.

During his confirmation hearings to be attorney general in January 2017, Sessions did not disclose his contacts with Kislyak; he later said the meetings were not about the Trump campaign. Sessions’s statements during his confirmation hearings raised questions about whether he had been truthful to the Senate. They also played a role in his decision in March 2017 to recuse himself from overseeing the Trump-Russia inquiry.

Other contacts raise additional ominous questions about the depths of the Trump campaign’s connections with Russia. At the National Rifle Association’s annual convention in May 2016, for example, Donald Trump Jr. spoke to Alexander Torshin, an official at the Russian central bank with close ties to Russian President Vladimir Putin. In January, McClatchy reported that the FBI was investigating whether Torshin illegally funneled money to the NRA in order to help Trump win the presidency. Torshin was one of a group of Russian individuals hit with new Treasury Department sanctions last week, an action that appears to have been taken independently of both Mueller’s investigation and the White House.

It has become increasingly clear that Mueller is aggressively pursuing questions about whether illegal Russian cash infusions helped Trump get elected.

Torshin is a life member of the NRA, as is a woman who was previously his special assistant at the Russian central bank, Maria Butina. Butina has been described in the press as a strong advocate for gun rights in Russia and has developed close ties to Republican Party operatives in the U.S. Paul Erickson, a longtime Republican operative who has fundraised for the NRA, formed a company in South Dakota with Butina and emailed a Trump campaign aide in 2016 about the idea of setting up a meeting between Putin and Trump.

Under normal circumstances, the fact that an FBI investigation was underway to determine whether the presidency had been won with Russian cash funneled through a highly partisan organization like the NRA would be enough on its own to spur talk of impeachment. But with Trump, it is just one more piece in a much larger mosaic of potentially illegal or even treasonous activity. Since the news of the NRA-Russia link broke, it has become increasingly clear that Mueller is aggressively pursuing questions about whether illegal Russian cash infusions helped Trump get elected. Mueller’s team has begun to question Russian oligarchs as they travel in the United States, stopping one when his private plane landed in New York. Mueller’s investigators have asked the Russians if they gave cash donations directly or indirectly to Trump’s campaign or his inauguration, CNN reported this month. Mueller has subpoenaed the Trump Organization for records related to its business with foreign nationals, including several Russians, the Times reported Monday.

Perhaps the most infamous meeting between the Trump campaign and Russians took place in June 2016, at Trump Tower in New York, when Donald Trump Jr., Manafort, and Jared Kushner, Trump’s son-in-law, met with Natalia V. Veselnitskaya, a Russian lawyer.

Explanations for why the meeting occurred have continually shifted ever since the New York Times first reported on it last year. Eventually, it became clear that Veselnitskaya had gone to the meeting with a memo she believed contained damaging information about the Democratic Party and Hillary Clinton. The Times reported that Veselnitskaya had discussed the allegations contained in the memo with Russia’s prosecutor general, Yury Chaika, and that her memo was similar to a document Chaika’s office had produced.

Former Trump campaign official Rick Gates leaves federal court on Dec. 11, 2017, in Washington, D.C.

One of the most intriguing connections between the Trump campaign and Russia was only recently revealed in documents released by Mueller’s team. Investigators have found that Manafort, the former campaign manager, had a business associate in Ukraine, Konstantin Kilimnik, who had connections to Russian intelligence that continued during the 2016 presidential campaign. Kilimnik was in contact with Manafort and his deputy, Rick Gates, during the campaign, and Gates knew that Kilimnik was a former officer with Russian military intelligence, according to the court filings. (Gates pleaded guilty in February and is now cooperating with Mueller’s investigation.) Kilimnik was still working with Russian intelligence while he was communicating with the Trump team in September and October 2016, according to the court filings.

Now there are new signs that Mueller has an important ally in his investigation of the Manafort-Gates-Kilimnik collusion evidence. Deputy Attorney General Rod Rosenstein, who oversees the Mueller probe because of Sessions’s recusal, wrote a memo last August saying that Mueller should investigate allegations that Manafort colluded with Russian government officials to interfere in the 2016 election. Rosenstein’s memo was made public earlier this month.

The Kilimnik connection has already led to jail time for one figure in the collusion case. Earlier this month, Alex van der Zwaan, a Dutch lawyer who worked at the firm Skadden, Arps, Slate, Meagher & Flom, was sentenced to 30 days in prison and a $20,000 fine for lying to the FBI about his communications with Gates and Kilimnik. Gates and van der Zwaan both communicated with Kilimnik during the 2016 election campaign, according to court documents.

The evidence of continued contact between the Trump team and the Russians after the election has also continued to build and now seems to be of real significance in Mueller’s investigation. American intelligence learned about a deeply suspicious December 2016 conversation between Kushner and Kislyak, the Russian ambassador. The two men reportedly talked about setting up a secret and secure communications channel between the Trump transition team and Moscow using Russian facilities. Maybe Kushner was just showing his naiveté. But his proposal to create a secret communications channel with Moscow using Moscow’s own secure communications systems — presumably so U.S. intelligence couldn’t eavesdrop — sounds like something a spy would suggest.

At about the same time, Gen. Michael Flynn, Trump’s first national security adviser, was also in repeated contact with Kislyak. In December 2016, before Trump took office, Flynn and Kislyak discussed sanctions imposed on Russia by the outgoing Obama administration. Flynn allegedly lied to Vice President Mike Pence about the content of that conversation and was subsequently fired. In December 2017, he pleaded guilty to lying to the FBI about his contacts with Kislyak and began cooperating with Mueller’s investigation.

Kirill Dmitriyev, CEO of the Russian Direct Investment Fund, left, and crown prince of Abu Dhabi Mohammed bin Zayed bin Sultan Al-Nahyan at a meeting of Russian government officials, members of the fund’s International Advisory Board, and foreign investors.

One post-election meeting that has attracted special interest from Mueller’s team took place in the Seychelles in January 2017, three months after the election. The existence of the meeting has been known since early last year, but details about its purpose have continued to trickle out, making the original explanations look less and less credible.

The meeting included Erik Prince, the founder of Blackwater, whose sister Betsy DeVos is Trump’s education secretary. Prince, who has been close to Trump’s national security team, met with Kirill Dmitriev, a Russian fund manager with ties to Putin, along with Mohammed bin Zayed, crown prince of Abu Dhabi and the de facto ruler of the United Arab Emirates, where Prince has had extensive business dealings. George Nader, a Lebanese-American businessman who also attended the meeting, is said to be cooperating with Mueller’s team, and has reportedly revealed that the meeting was designed to create a backchannel between the new Trump administration and Moscow.

Joseph Mifsud in 2014.

While evidence of collusion continues to pile up, I keep coming back to the curious case of Joseph Mifsud. Anyone who has read John le Carré’s spy novels would immediately recognize him. The slightly sketchy intermediary between East and West is a le Carré archetype. In his 1979 novel “Smiley’s People,” there is Otto Leipzig, nicknamed “The Magician,” who blackmails a Russian intelligence officer to get vital information to the British, but ends up dead.

Le Carré could hardly have invented a better go-between than the Maltese professor.

Le Carré could hardly have invented a better go-between than the Maltese professor.

Mifsud has spent years as a nomadic academic, with postings of uncertain seriousness at institutions throughout Europe, including several that seem to have occurred at more or less the same time. He has variously been described as a teaching fellow at University of Stirling in Scotland; the “honorary director” of the London Academy of Diplomacy; an “honorary” professor at the University of East Anglia; and a visiting professor at Link Campus University in Rome. At one time, he reportedly held a position with the London Centre of International Law Practice, where George Papadopoulos also worked for a few months as director of the organization’s Center for International Energy and Natural Resources Law and Security.

Mifsud’s academic life and background would provide perfect cover for an intelligence asset. His credentials were just enough to gain him entree to the European academic circuit, yet his postings couldn’t withstand much scrutiny once his name surfaced in the Trump-Russia case. Quartz quickly discovered that the “London Centre of International Law Practice” was nothing more than “four people working in an undecorated backroom, all of whom declined to comment.”

Russia’s President Vladimir Putin speaks at a plenary session titled “The World of the Future: Moving Through Conflict to Cooperation” as part of the 14th annual meeting of the Valdai Discussion Club in Sochi.

In recent years, Mifsud sometimes made appearances at the Valdai Discussion Club, a Russian think tank that is widely considered in the West to be pro-Putin. So pro-Putin, in fact, that the Russian president regularly addresses the club in person; Mifsud noted publicly that he had attended at least one of Putin’s speeches at the Valdai club. Mifsud published several papers that appeared on the Valdai website, including a fawning 2015 piece of pro-Putin propaganda about Russian policies in Syria.

“Whilst according to many Western and Arab analysts the United States has been dillydallying with the world order, more intent on imposing a verbal “unilateral diktat”, other centres of power (e.g. the Russian Federation in Syria, Saudi Arabia in Yemen) were taking the bull by the horns and resorting to military force as a measure of last resort,” Mifsud wrote. “One can also contend that the Russian military participation in Syria was clearly organised in such a way as to ensure that diplomacy also takes its course. … The United States administration is on the defensive, still heavily labouring under the internal repercussions of the nuclear deal with Iran, the ‘fiasco’ in Libya, the heating up of the presidential race, the dismantling of the ‘iron partnerships’ with Saudi Arabia and Israel, and the overt economic power struggle with China. On the other hand, the Russian leader has walked the talk with ‘facts on the ground’ and has ensured that the Russian presence emanating from its direct involvement in the war in Syria will have a major impact on the shared solution.”

After Papadopoulos’s guilty plea became public last October, an Italian newspaper quickly tracked Mifsud down in Rome. “This is nonsense,” Mifsud said, referring to Mueller’s accusations that he had acted as an intermediary between the Trump campaign and Moscow. “The only thing I did was to facilitate contacts between official and unofficial sources to resolve a crisis. It is usual business everywhere. I put think tanks in contact, groups of experts with other groups of experts.” He denied “any discussion of mine about secrets concerning Hillary Clinton. … Let’s be clear: the Russians didn’t ask me to meet Papadopoulos.”

Shortly after that interview, published on November 1, 2017, Mifsud disappeared. No one has acknowledged seeing him since. BuzzFeed reported last month that Italian prosecutors, who were seeking him in an unrelated case, couldn’t find him.

Mifsud’s fiancée, Anna, recently gave birth to a baby girl. She says Mifsud is the child’s father, yet she still hasn’t heard from him. Questions abound about what happened to the professor. But one thing we know is that a key intermediary between the Trump campaign and Moscow has been missing for months. Mifsud’s disappearance comes at a time when bad things are happening to those who get in Vladimir Putin’s way.

That makes me increasingly suspect that Mueller’s investigation into collusion is on the right track.