Last week, the leader of the British Labour party, lagging behind Boris Johnson’s Conservatives in all polls ahead of the imminent election, played what he believed to be his trump card. He presented leaked documents indicating that the United States would demand more access to the country’s beloved National Health Service in any trade talks after Brexit.

The theory, as often repeated by Jeremy Corbyn but strongly denied by both President Donald Trump and Johnson, goes that the prime minister is plotting to sell access to vast swaths of the sacred health service to U.S. pharmaceutical firms to help him see through a trade deal with Washington. The leaked documents finally seemed to add at least some evidence that the suspicions should be taken seriously ahead of the Dec. 12 vote.

But there’s a problem. The documents brandished by Corbyn were, within hours of his announcement, shown to have been posted weeks earlier by an anonymous user on Reddit. Now experts say the leak and distribution of the classified British-U.S. trade documents are eerily similar to a disinformation campaign uncovered this year that originated in Russia.

The experts—researchers at Britain’s Oxford and Cardiff universities, the Atlantic Council think tank, and analytics firm Graphika—aren’t saying the documents are fake or accusing Labour of working with the Kremlin. However, they have said that the way the documents were first shared with the public on Reddit replicated a known tactic dubbed Secondary Infektion.

Secondary Infektion was first uncovered by the Atlantic Council in June. The campaign used fabricated or altered documents and spread the false information across online platforms. It all stemmed from a network of social-media accounts which Facebook confirmed “originated in Russia.”

The researchers now say the sites that were used to put the U.S.-U.K. trade documents online; the way they were subsequently shared on Twitter; and the clunky language errors in the posts accompanying the papers all made it resemble the Secondary Infektion disinformation campaign.

Ben Nimmo, the head of investigations at Graphika, said in the report: “What we are saying is that the initial efforts to amplify the NHS leak closely resemble techniques used by Secondary Infektion in the past, a known Russian operation. But we do not have all the data that allows us to make a final determination in this case.”

The NHS documents were first posted on Reddit by a user called Gregoriator, who, according to the researchers, made the same kind of grammatical errors that were made by those behind Secondary Infektion. For example, the post said the documents showed that “Great Britain is practically standing on her knees” in seeking a U.S. trade deal.

A Twitter account using the @Gregoriator username then tried to share the leak by tweeting a link to the original Reddit post to a wide range of British politicians, journalists, and celebrities such as Stephen Fry. Weeks later, the document fell into the hands of staffers at the Labour party and were presented publicly at a Corbyn press conference.

Labour has now come under pressure from senior Conservative party figures to “come clean” on how it obtained the tainted documents. Asked to explain during a Tuesday morning TV interview, Corbyn said: “I held the dossier up because it had been released and I’d seen it and at no stage up until today has anyone challenged the veracity of that document.”

Graphika’s report concluded that the critical question to answer now is how the unredacted documents ended up in the hands of the anonymous person or people who posted them on Reddit. It raises the potentially explosive possibility that someone inside the British establishment may have leaked the potentially election-changing documents to foreign actors.

The Kremlin ridiculed suggestions that Russian hackers leaked British-U.S. trade documents. “It’s very convenient for demonization, to cover up one’s own headache, and to use this fetish to frighten people with Russian hackers. We have repeatedly come up against this and we view it with a dose of irony,” Kremlin spokesman Dmitry Peskov said Tuesday.

However, the researchers are adamant. “Whoever did this... was absolutely trying to keep it a secret,” said Graham Brookie, director at the Atlantic Council’s digital research lab. “It carries the specter of foreign influence.”