"The Greek law enforcement received intelligence on plans to prepare an assassination via poisoning with the help of criminals. The head of the prison and the prosecutor of the city of Thessaloniki summoned Vinnik and informed him about the plot to poison him, and special security measures were taken in connection with that," the source said.
A Sputnik correspondent saw documents confirming the murder plan reports.
Vinnik has been forbidden from receiving any things, food or water from any people he did not know, his guard has been enhanced, and he is only allowed to move around the prison with beefed up security around him so that he would not come in contact with other inmates, the source added.
The police received information about the murder plot at the beginning of this year, but it was not made public to help the investigators.
According to the source, no special services of any country are connected to the assassination plot, the murder attempt is linked to the criminal world.
"It all began after Vinnik's extradition to the United States was blocked. There people who are extremely interested in him not coming to Russia. The assassination was ordered by some unknown person from Russia," the source said.
The source added, when asked about similarities, that there was no connection between this case and the case of former Russian spy Sergei Skripal, poisoned in the United Kingdom.
"Vinnik wrote an acknowledgment of guilt in Russia, which says he knows about crimes committed in the sphere of financial technologies. Vinnik said he is ready to give testimony in Russia and assist the investigation," the source said.
Vinnik's lawyers declined to comment on the reports of a murder plot.
"Such statement should be investigated via a proper procedure, rather than through public statements and discussions," one of the Russian national's lawyers told Sputnik.
In October, two different judge panels in Thessaloniki adopted two mutually exclusive decisions of the same judicial force, one on the extradition of Vinnik to Russia and the other on his extradition to the United States. In December, the Supreme Court of Greece upheld the decision to extradite the Russian citizen to the United States. In January, Vinnik’s lawyer Timofey Musatov told Sputnik that Vinnik would appeal the extradition ruling with the European Court of Human Rights.