Dozens of athletes expecting to compete at the coming Summer Olympics in Rio de Janeiro could be barred from the Games, the International Olympic Committee announced on Tuesday.

The I.O.C. retested 454 doping samples from the 2008 Summer Games in Beijing, it said, discovering suspicious results among 31 athletes from 12 countries and six sports. The athletes were not publicly identified on Tuesday. The countries in question will be notified in coming days, according to a spokesman.

An additional 250 doping samples from the 2012 Summer Games in London are due to be retested, officials said, announcing broad scrutiny of athletes who have competed in recent Olympic Games.

The announcement came in the wake of a detailed account last week by the former longtime director of Russia’s antidoping lab, Grigory Rodchenkov, who said he worked for years at the direction of the Russian government to help the country’s top athletes use banned, performance-enhancing substances and go undetected.