LONDON—One of the Brexit campaign chiefs appeared to pass documents detailing an American law enforcement investigation to a Russian official, according to a cache of leaked emails.

The papers, which detailed a probe into dark web money laundering, were apparently shared with the Russian embassy in London by Leave.EU executive Andy Wigmore. They concerned the arrest of Brexit financier George Cottrell, who was seized at an airport on the way home from the Republican convention in 2016 where Donald Trump had just been nominated as the presidential candidate.

Cottrell had been at the convention in Cleveland with his boss Nigel Farage, who dined with Roger Stone and met a string of other Republican operatives and elected officials.

The young financier, who led Brexit fundraising for the U.K. Independence Party, was ultimately convicted of offering to launder money across international borders on the dark web before he joined UKIP.

A Daily Beast investigation of Cottrell revealed that he had a series of online links to small banks that were part of the notorious Russian Laundromat scam, which allowed dirty money to be shipped all over the world. A UKIP insider wrote that it was his knowledge of the “murky and complicated world of shadow banking” that “landed Cottrell an unpaid role” in the party.

UKIP officials said they had no idea that Cottrell had touted himself as a money launderer on a TOR black market site until they saw the indictment against him.

Emails obtained by The Observer show that Wigmore passed that indictment to a contact at the Russian embassy.

One of the emails, seen by The Daily Beast, had the subject line “Fwd Cottrell docs — Eyes Only,” it had six attachments. The message was sent to Sergey Fedichkin, a political officer at the embassy, on August 20, 2016, a day after Cottrell appeared in court in Phoenix, Arizona, with the message, “Have fun with this.”

The Russians would have been able to obtain the documents themselves but Wigmore’s willingness to share the papers with the Russian diplomat indicates the depth of the relationship between Leave.EU and the Kremlin’s men.

During his appearance before a parliamentary committee, Wigmore told Members of Parliament that he had never discussed Cottrell's arrest with the Russian embassy. “It never came up. While at the time it probably seemed a big thing, there was so much else going on at the time it just was not an issue,” he said.

Wigmore and his Leave.EU colleague Arron Banks originally claimed that they had no relationship with Russia during their successful battle to take Britain out of the European Union.

In Bank’s diaries, The Bad Boys of Brexit, he admitted that he, Wigmore and Banks’ Russian wife had enjoyed a boozy lunch at the Russian embassy during the campaign but they claimed it had been a one-off.

Banks, who was by far the biggest financial backer of Brexit, has faced a swirl of questions about the origin of the millions of dollars he spent on the campaign. Last week it emerged that he and Wigmore had a lengthy relationship with Russian officials, that included a trip to Moscow. Emails published in The Observer suggested that Banks and Wigmore had also been given the potential opportunity to invest in a Russian gold mine.

It is unclear why Wigmore felt the Russian embassy should be kept in the loop on the IRS undercover agents’ sting which had snared Cottrell. Although he was co-director of Brexit funding for UKIP, Cottrell was not a well-known figure at the time.