Vinnik was arrested in Greece while vacationing with his family in 2016, after Swedish software engineer Kim Nilsson, who had been investigating Mt. Gox's missing bitcoins, tracked several of the missing coins to a wallet owned by Vinnik.
Vinnik was accused by the U.S. of helping criminals circumvent KYC/AML policies in order to launder money with bitcoin; specifically, he was arrested in connection with the hacking attacks on Mt. Gox that caused the Tokyo-based exchange to lose millions of dollars worth of its customer's bitcoins.
While Vinnik claims he was not guilty of orchestrating the hacking attacks, he is the only employee of the exchange who has been accused of any charges relating to the Mt. Gox incident. Vinnik claimed in 2017 that he did not orchestrate the attacks and had no knowledge of criminal activity on BTC-e, and that he was only a technician. He also denied the claim that he was the exchange's operator. He appealed to Greek authorities for extradition to his native Russia for trial. The United States appealed for Vinnik to be extradited to the U.S., which was greenlit by the Greek court system. Russia responded to the verdict with criticism, calling the verdict "unjust," and that it violated international law. The Russian government added that it hoped Greece would reconsider its request to have Vinnik extradited to Russia.
This caused a stalemate that effectively blocked Vinnik's extradition to the U.S. Vinnik allegedly wrote an admission of guilt to Russian authorities, vowing to testify in his native country. In May 2018, Greek authorities uncovered a plot to murder Vinnik in prison that was allegedly "linked to the criminal world." France later filed a similar extradition request as the U.S. - claiming Vinnik had laundered 133 million euros using bitcoin - which was approved on July 13, 2018. However, in September, Greece's supreme court announced it had approved Vinnik's extradition to Russia, with a written statement to follow September 14.
Vinnik filed an appeal in a Piraeus city court for release or extradition to Russian. In the appeal, he said this was due to "humanitarian reasons."
Possible connection to Mueller investigation
After news of the extradition reached the U.S., Bloomberg reported that based on evidence collected by the United States Federal Beureau of Investigation, Vinnik could have a connection to the Russian hackers being investigated by U.S. Special Counsel Robert Mueller, who is investigating the influence of Russian hackers on the 2016 U.S. elections. Though Vinnik himself is not suspected of directly influencing the elections, he may have helped Fancy Bear, a Russian hacking unit linked to the attacks, launder money using bitcoin.