The man who called for Russia's ban from international athletics says it will be "hard" to believe its athletes are clean if they compete at Rio 2016.
Dick Pound, ex-president of the World Anti-Doping Agency (Wada), led the commission which found the country's doping problem was "widespread".
Russia was later banned but must prove it is clean to go to the Olympics.
"There's an onus on them to show that whatever changes are required have indeed been effected," Pound said.
Athletics' world governing body the IAAF is meeting in June to discuss Russia's re-admission in time for the Olympics, which start in August.
Speaking at a Sporting Resolution Conference in London on Thursday, Pound said: "I think there's some elements of denial but at least the president of the federation has made it clear to all of the people in the sports structure that there is a problem that has to be addressed."
But he added there would remain some suspicion of Russian athletes even if the IAAF is convinced to let them compete at the Games.
"If we let the Russians back in, are we absolutely certain that every Russian athlete isn't doping and everything has changed?"
"I think it's very hard to say that."
Former British distance runner Paula Radcliffe, who also spoke at the conference, said it was a no-win situation.
"Somewhere in all of this innocent athletes are going to lose out somehow, whether it's the innocent athletes who compete against the cheats who are allowed back in, or whether it's the ones who are banned because some of their countrymen, or a lot of their countrymen, were cheating," she said.
"We're all suspicious that they can do what they need to do to assure us the entire team is clean to compete fairly in Rio."