In an interview published on Saturday by Germany's Der Spiegel, the whistleblower behind the "Panama Papers," which brought to light massive tax evasion and fraud around the world, said that he fears Russian retaliation.
Using the pseudonym John Doe, he spoke to the magazine and said that he had evidence of financial wrongdoing by Russian officials and their allies who financed the war in Ukraine.
Spiegel, when asked if he feared for his life, said, "It's a risk that I live with, given that the Russian government has expressed the fact that it wants me dead", AFP reported.
Spiegel asked John Doe about tax havens used by "strongmen in autocratic regimes," to which he replied about the role they play in Russia, whose leaders deny breaking the law.
Russian President Vladimir "Putin is more of a threat to the United States than Hitler ever was, and shell companies are his best friend," he said.
"Shell companies funding the Russian military are what kill innocent civilians in Ukraine as Putin's missiles target shopping centres."
He said anonymous firms "make these horrors and more possible by removing accountability from society. But without accountability, society cannot function."
He spoke about a two-part docudrama aired by the Russian state-funded channel RT featuring a "John Doe" character who "suffered a torture-induced head injury during the opening credits."
"However bizarre and tacky, it was not subtle," he said.
"We have seen others with connections to offshore accounts and tax justice resort to murder, as with the tragedies involving Daphne Caruana Galizia and Jan Kuciak," speaking about the investigative journalists killed in Malta and Slovakia.
In his first interview since the release of the Panama Papers in 2016, John Doe said he has no plans to come out of anonymity.
"The Panama Papers involve so many different transnational criminal organizations, some of them with links to governments, that it's difficult to imagine how it could ever be safe to identify myself," he said.
The Panama Papers are one of several financial documents leaked by the International Consortium of Investigative Journalists (ICIJ).
Their revelations led to the resignation of the Prime Minister of Iceland and led the way for the ousting of a Pakistani leader.