In an interview with ABC News, Valery Morozov said he was told he would be "drowned in blood" for accusing officials in the administration of President Vladimir Putin of demanding payoffs in the construction of venues and roads for the 2014 Winter Olympics.
Morozov first made the accusation in 2010, and has since moved to the London area.
Boris Nemtsov, a Putin rival who formerly served as deputy prime minister, told ABC News he has evidence Russian officials and business executives have stolen as much as $30 billion of the estimated $50 billion to $60 billion spent on Olympics-related projects -- which he said has helped friends of Putin become billionaires while sticking the Russian public with the bill for Sochi.
"Everybody knows this is the most criminal case in the history of Russia," he said.
A former Russian Olympic Committee vice president fled the country after being charged with embezzlement, a charge he denied, ABC reported.
"We have not seen any big, large-scale instances of corruption in connection with the implementation of Sochi Olympics project," Putin said in a recent interview with ABC.
The International Olympic Committee says corruption is a matter for the Russian government to deal with.