I don’t know this for a fact, but I can surmise it from what I’ve learned. At some point, Robert Mueller’s investigative team put together a list of foreigners who were placed on a special watchlist. Every day as people from around the world travel to the United States, their names are scanned and if someone on Mueller’s list pings the system, the FBI has teams ready to intercept them at customs.
They are pulled out of line and questioned exhaustively. All their electronic devices are temporarily seized and their information is recorded. It’s likely that, once released, their movements and activities within the country are shadowed and memorialized.
The latest example is Stephan Roh, a German (or perhaps Swiss) citizen and multimillionaire whose wife, Olga Roh, is “a Russian fashion designer who appeared on the British reality TV show Meet the Russians.” According to a book Roh co-authored that is scheduled for publication in June, he was detained at JFK airport in October 2017 and interrogated about his relationship with Maltese Professor Joseph Mifsud. (For background, I’ve written about Professor Mifsud extensively. See here, here, here, here, and here).
The Cliff Notes on Mifsud are that he befriended George Papadapoulos in Italy after learning that he had a foreign policy advisory position with the Trump campaign. They subsequently met up repeatedly in London where Mifsud introduced Papadapoulos to a woman purporting to be Vladmir Putin’s niece. He also connected Papadapoulos to a contact at the Russian Ministry of Foreign Affairs.
Through Mifsud, Papadapoulos learned very early on that the Russians had acquired a large trove of Hillary Clinton’s emails and were planning to use them to damage her campaign. One night after many drinks, Papadapoulos confided this information to Australia’s ambassador to the United Kingdom. The ambassador duly reported this back to his headquarters but nothing was done with the information until after emails from the Democratic National Committee were released. Armed with the information that Papadapoulos had foreknowledge of the crime, the FBI began a counterintelligence investigation in the summer of 2016.
Mifsud had a presence in the United Kingdom where he ran the London Academy of Diplomacy and had a professorship at University of Stirling in Scotland. But he also was a professor at Link Campus University in Rome. That’s his primary connection to Stephan Roh.
Roh intersected with Mifsud at two institutions: the now-defunct London Academy of Diplomacy and Link Campus University, a private institution in Rome that Roh co-owns, and where Mifsud taught briefly. In April 2016, Mifsud and Roh spoke on a panel together at the Kremlin-backed Valdai Club—a think tank that is close to President Vladimir Putin and hosts him every year for a keynote address. The club is described in the book as “one of the most influential Russian think tanks in Moscow, maybe even the most prestigious.”
For a background on Mifsud’s career and extensive ties to Russia, I recommend this piece by Brian Whitaker and this piece by Alberto Nardelli for BuzzFeed that focuses on Mifsud’s pregnant Ukranian girlfriend.
Mifsud has been at the center of the Russia investigation since its inception, but Stephan Roh’s name has come up only tangentially until now. One interesting tidbit you may be interested in is that the same month, October 2017, that Mr. Roh was being questioned at JFK airport, Prof. Mifsud was in Moscow:
Mifsud visited Moscow in October 2017 as a member of a Saudi delegation headed by King Salman, according to a report by Alberto Nardelli for BuzzFeed. Mifsud is said to have taken part in a seminar about security challenges in Yemen organised by the Russian Council of International Affairs (RIAC) and the King Faisal Centre for Research and Islamic Studies.
What makes this especially curious is that most reporting on Mifsud suggests that he’s been “missing” or underground. For example:
Mifsud has virtually disappeared since his name was made public late last year. In their book, Roh and Pastor say that “the head of the Italian secret services contacted the President of LINK Campus, Vincenzo Scotti,” and recommended that Mifsud “disappear.” Since then, Mifsud “has been requested to hide, not to communicate, and not to speak to the press,” Roh and Pastor write. “He has been ‘put away’ and threatened to stay quiet.”
According to Roh’s book:
…Mueller interviewed another Mifsud associate in the summer of 2017: Ivan Timofeev, a program director at a Russian government-funded think tank who Mueller described in court filings as “connected to the Russian Ministry of Foreign Affairs.” Mifsud connected Timofeev to Papadopoulos by email in the spring of 2016, according to the filings. Over lunch earlier this year, the book says, Timofeev described being stopped by the FBI at JFK in “mid-2017” and questioned about his relationship with Papadopoulos, the DNC hacks, and about the “thousands of emails with dirt on Hillary Clinton.” His cellphone and laptop were seized, too.
According to Mueller’s indictment of Papadapoulos, Ivan Timofeev became George Papadapoulos’s handler. 
Basically, Mifsud recruited and compromised Papadapolous and then handed him over to Timofeev. From there, Papadapolous was strung along with promises of being able to arrange (and take credit for) a one-on-one meeting between Donald Trump and Vladimir Putin. In the following screenshot, the “individual in Moscow” who says he has connections at the Russian Ministry of Foreign Affairs is Timofeev. It may not be properly appreciated by most people covering this investigation that Mueller detained Timofeev at JFK airport in mid-2017 and copied all the information on his cellphone and laptop.
As for why Stephan Roh was detained, at the time that Mifsud emailed Papadopoulos (April 18, 2016) to introduce him to Timofeev, he was in Moscow for a panel discussion on global energy at the Kremlin-linked Valdai Club. Look at who else was there:
The panel consisted of Mifsud, Professor Igor Tomberg of the Russian Academy of Sciences, and Stephan Roh, a Swiss lawyer who officially resides in Monaco. A short report of the discussion appears on the Valdai website.
Chairing the discussion was Ivan Timofeev, who is programme director of Valdai and also programme director of the Russian International Affairs Council (RIAC) — a think tank established in 2010 by presidential decree.
A video apparently recorded on the same day shows Timofeev interviewing Misfud about energy.
It’s not difficult to understand why Stephan Roh and Ivan Timofeev were on Mueller’s special watchlist. It’s a bit harder to understand why Russians on the watchlist continue to travel to the United States with their cellphones and laptops. So far, we’ve been discussing one small but hugely significant corner of the investigation, but Mueller has been detaining others, too, including Russian oligarchs tied to Michael Cohen or Paul Manafort. Some non-Russians have been detained and arrested, like George Nader and Papadopoulos.
Because Mueller can seize electronic devices from foreigners at points of entry, and because people keep showing up at our airports with their electronic devices in hand, Mueller must have acquired an extraordinary amount of information about a wide range of criminal activities, only some of which will have anything to do with the 2016 election.
The tactic gets less effective every time it is reported to have been utilized, yet people continue to walk into the trap.
It’s safe to say that we haven’t seen the tip of the iceberg of the kind of information Mueller has acquired. But I imagine that the simple task of telling the story will take time. Anna Karenina wasn’t written in a day.
