(CNN) - The strange series of events that have followed US President Donald Trump's one-to-one meeting with his Russian counterpart Vladimir Putin in Helsinki on Monday has given rise to debate in Washington and in the media over whether Moscow does indeed have incriminating intelligence that it's holding over Trump's head.
There has so far been no evidence to support that case.
But Steve Hall, a former CIA station chief in Moscow, issued a chilling warning on CNN's "Anderson Cooper 360" on Thursday: "Before the Helsinki summit, I was not prepared to go to the darkest corner in the room and say there is kompromat -- there is compromising information -- on Donald Trump."
"After ... I saw Donald Trump treat (Putin) in a fashion that is just inexplicable, the only conclusion that I can come to is ... I think there is information and data out there that implies there is indeed compromising information that Vladimir Putin has on Donald Trump. Why else would he treat him that way?"
The gathering of kompromat -- embarrassing material intended to be used against someone -- is a well-known tactic in Russia, according to former British Ambassador to Russia Tony Brenton.
"It is very much a part of the way Russia works, that intelligence agencies collect compromising information on individuals and that they'll use it when it's to their advantage," he told CNN.
The practice is so widespread that Brenton says British diplomats posted to Russia are warned to be on their guard for it as part of their pre-assignment security briefings.
"It is very likely that Russia will collect dossiers of compromising material about US politicians. However, I think it is highly unlikely that these dossiers would find their ways into western hands," he says.
James Nixey, head of the Russia and Eurasia program at Chatham House, says the use of financial or sexual kompromat "used to embarrass, expose or keep obedient" began in the Soviet era.
"The fact of the matter is most people have a file, most people who have been to Russia of any commercial or political significance have a dossier squirreled away on them," he says.
"This is done elsewhere, but it's not leveraged for political or financial gain in the same way as it is in Russia."
Igor Sutyagin, senior research fellow at London's Royal United Services Institute, says that to understand kompromat, you must first understand Russia's political culture.
"It's standard for Russian politicians to gather kompromat on all members of their inner circle, it's a matter of survival," says Sutyagin. "It's not only possible, it is absolutely normal. Putin grew up in this environment, so it's absolutely natural for him to operate in this way, to gather kompromat."
Sex and sexuality has been one of the key areas of exploitation in the past.
Cases of kompromat
British civil servant John Vassall was targeted while working at the British embassy in Moscow in the 1950s. Vassall was gay at a time when homosexuality was illegal in the UK.
"The Russians ... found the chink in my armor before anyone else," he later explained.
In his autobiography, "Vassall: The Autobiography of a Spy," he recalled being shown images of "a party I could not believe I'd been at. There I was, naked, grinning into the camera ... after about three photographs, I could not stomach anymore."
The incriminating pictures, of him "enjoying every possibly sexual activity ... with a number of different men," were used to convince him to spy for the KGB.
When Vassall's activities on behalf of the Soviet Union were revealed at the height of the Cold War, the news rocked the British government. Vassall was later sentenced to 18 years in prison.
The use of kompromat didn't end with the Cold War and the fall of the Berlin Wall.
In 1999, Yuri Skuratov, Russia's then Prosecutor-General, was in the middle of one of the biggest investigations of his career, tracking down Russian high-profile officials accused of taking huge bribes, when a tape surfaced.
Grainy footage, apparently captured on a concealed camera, showed a man who bore a resemblance to Skuratov having sex with two prostitutes.
Hours after it aired on Russian state television, Skuratov was suspended from his post, despite his protestations -- widely reported at the time -- that "the case against me is fabricated."
Igor Sutyagin says it was Putin, who was working at the Federal Security Agency (FSB) at the time, who proved his allegiance to then President Boris Yeltsin by helping to destroy Skuratov's career.
Putin publicly certified that Skuratov was the man pictured on the videotape. Yeltsin appointed Putin prime minister in the same year.
This wasn't the only case in which questions were raised about whether the incriminating material involved was genuine.
In 2009, a tape claiming to show Moscow-based US State Department employee Brendan Kyle Hatcher having sex with a prostitute surfaced on the Compromat.ru website.
Hatcher denied the allegation and the then State Department spokesman Ian Kelly described the "campaign ... to smear a foreign service officer" as deplorable.
"Clearly, the video we saw was a montage of lot of different clips, some of them which are clearly fabricated," the then US ambassador John Beyrle told ABC News.
While all three of these cases made international headlines, Tony Brenton and James Nixey say most kompromat operates invisibly, with people threatened quietly.
"Information is only useful, for the most part, if it's not being used," says Nixey. "The most useful stuff is the stuff we don't hear about, by its very nature."
"The very fact that there is a specific Russian word for this" is emblematic of how all-pervading the practice of kompromat is in Russia, he added.
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