Hundreds of doping test samples of athletes from the Beijing and London Olympics are being reanalysed ahead of Rio 2016.
The International Olympic Committee and the World Anti-Doping Agency will use techniques that have advanced since those events in 2008 and 2012.
The aim is to find athletes who cheated at those Games, and prevent them from competing in Brazil.
The IOC Medical and Scientific Director Richard Budgett says: "The aim of the programme is to prevent athletes who cheated in London or Beijing, and got away with it because we didn’t have as advanced methods of analysis as we do now, from competing in Rio de Janeiro."
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