(CNN)As the largest backer of the campaign for the UK to leave the European Union, Arron Banks rejoiced in being called one of the "Bad Boys of Brexit." Now the UK's National Crime Agency is investigating his financial support for the campaign and his contacts with Russian officials, according to three sources familiar with the probe.
UK investigates alleged Russian links to Brexit campaign
The agency recently received an extensive archive of Banks' emails and other documents, the sources say, which include descriptions of meetings with Russian diplomats and possible business opportunities in Russia.
At least some of those emails have already been leaked to media organizations in the UK and the US. Banks, who provided loans and donations totalling about $12 million to Leave.EU, the Brexit campaign group he co-founded, has complained to the police that they were stolen.
A spokesperson for the National Crime Agency told CNN it never confirms or denies the existence of investigations. The UK's Serious Fraud Office also declined to comment on whether it had opened an investigation. The Sunday Times of London first revealed the National Crime Agency's involvement last weekend.
In a text to CNN on Monday, Banks's spokesman Andy Wigmore said the NCA had not contacted either of them. "And they are unlikely to, firstly because both UK and US authorities were fully briefed and know the truth -- there is no evidence of any wrongdoing or collusion because it didn't happen so even they know there is nothing to investigate," Wigmore said.
Banks' financial support for Leave.EU in the runup to the 2016 vote made him the biggest individual donor in British political history. The group won the endorsement of the leader of the UK Independence Party, Nigel Farage, one of the leading pro-Brexit campaigners. According to the latest accounts of Leave.EU, Banks' loan has not been paid back.
His emails were passed to the National Crime Agency by a third party. Sources have told CNN the documents have also been shared with the UK's domestic intelligence service MI5 and, in the US, with Democratic members of the House Intelligence Committee. The committee has been investigating allegations of collusion between people assoc