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London — In a major doping crackdown stretching back eight years, 31 athletes in six sports could be barred from competing in this year’s Olympics after they were caught in retesting of drug samples from the 2008 Beijing Games, and other positive cases could emerge from the 2012 London Games.

The International Olympic Committee opened disciplinary proceedings Tuesday against the 31 unidentified athletes from 12 countries who competed in Beijing and were planning to take part in the Rio de Janeiro Games in August.

“This is a powerful strike against the cheats,” IOC President Thomas Bach said. “They show once again that dopers have no place to hide.”

The IOC said it also planned to reanalyze drug tests from the 2014 Sochi Winter Games after allegations samples were tampered with as part of a state-sponsored Russian doping program.

The positive cases from Beijing emerged from the recent retesting of 454 doping samples with “the very latest scientific analysis methods,” the IOC said.

The IOC stores samples for 10 years to allow for retesting with improved techniques, with athletes caught facing retroactive disqualification and loss of any medals.

The IOC said it could not identify the athletes caught in the Beijing retests for legal reasons, saying it would inform the relevant national Olympic committees in the coming days.

“All those athletes infringing anti-doping rules will be banned from competing at the Olympic Games” in Rio, the IOC said after a teleconference meeting of its policy-making executive board.

Results of retesting of 250 samples from the London Olympics will be announced shortly, the IOC said. Those tests were also aimed at athletes planning to compete in Rio.

The IOC said it would also undertake a “wider retesting program” of medalists from both the Beijing and London Games. Samples of athletes who could be promoted to medals following disqualification of drug cheats will also be retested.

The IOC also asked the World Anti-Doping Agency to launch a “fully fledged investigation” into allegations that the drug-testing system in Sochi was subverted by Russian officials.

The IOC said it would ask the Lausanne anti-doping lab and WADA to proceed with analyzing Sochi samples “in the most sophisticated and efficient way possible.”

It’s unclear, however, how many samples are still intact for reliable retesting. Grigory Rodchenkov, the former head of the Russian anti-doping laboratory, told the New York Times last week that he gave Russian athletes a cocktail of drugs before the Olympics and switched tainted urine samples for clean ones during the games. He has offered to assist in retesting.

The statute of limitations for retesting was extended in 2015 from eight to 10 years, meaning the Beijing samples remain valid through 2018.

It’s not the first time that samples from Beijing have been retested. A few months after those games, the IOC reanalyzed nearly 1,000 of the total of 4,000 samples with a new test for the blood-boosting drug CERA. Five athletes were caught, including 1,500-meter gold medal runner Rashid Ramzi of Bahrain.

Nearly 500 doping samples from the 2006 Winter Olympics in Turin, Italy, have already been retested. The IOC has not disclosed whether those retests had produced any positive cases.

Five athletes were caught in retests of samples from the 2004 Athens Olympics, including men’s shot put winner Yuriy Bilonog of Ukraine.

On a separate issue, the IOC said it would “work to shed full light” on bribery allegations tied to Tokyo’s winning bid for the 2020 Olympics.

French prosecutors said last week that 2.8 million Singapore dollars ($2 million) was apparently transferred from Japan to the Singapore account of a company tied to the son of former IAAF president Lamine Diack, who is facing corruption charges.

Tokyo bid leaders acknowledged the payments were made, but said they were for legitimate consulting fees.

The IOC said it would remain a civil party to the French investigation and its chief ethics officer would “actively cooperate” in the inquiry.

Q&A: Olympic doping scandal

Some questions and answers about the latest doping scandal in sports:

Q: How can athletes be caught for doping so many years later?

A: The International Olympic Committee keeps all Olympic doping samples for possible retesting. Samples are frozen and stored at the anti-doping laboratory in Lausanne, Switzerland. The original statute of limitations for retesting was eight years, but that was extended to 10 years in 2015. The IOC can retest samples with new and improved techniques to catch cheats who escaped detection at the time. Normally, the IOC prefers to wait until near the deadline so that it can make use of the very latest testing methods. In this case, the IOC decided to test selected samples from Beijing to weed out any cheats before they got to Rio, using “the very latest scientific analysis methods.”

Q: Which athletes were caught in the Beijing retests?

A: We don’t know yet. The IOC did not identify the athletes, their sports or their nationalities, citing legal reasons. It’s possible names could start leaking out once the various national Olympic bodies and the athletes are informed. In the meantime, some athletes will be getting nervous.

Q: What action can the IOC take against the athletes?

A: If athletes are found guilty of doping, they will be banned from competing in Rio. In addition, they could be retroactively disqualified from the Beijing Games and stripped of their results and any medals. Any longer-term punishments and suspensions are up to the individual sports federations.

Q: What’s the process now?

A: The IOC has not yet laid out how the procedures will work. Normally, if athletes are accused of a doping violation at the Olympics, they are entitled to ask for testing of the backup “B’’ sample and a hearing is held. With the Rio Olympics starting on Aug. 5, the process will have to take place swiftly.

Q: What about samples from the 2012 London Olympics?

A: The IOC has also been reanalyzing some London samples, with results of 250 retests “to come shortly.” Those tests, too, focused on athletes planning to compete in Rio. The IOC is also conducting a wider retesting of samples of medalists from Beijing and London, so there could be a major reallocation of medals to come from both those games.

Q: Will samples from the 2014 Sochi Winter Games be retested?

A: Yes. While the IOC could wait until 2024, it said it is taking “swift and decisive action” following allegations by former Russian lab director Grigory Rodchenkov that he was involved in state-sponsored doping in Sochi and manipulated samples to cover up cheating by Russian athletes, including 15 medalists. The IOC said it will work with the World Anti-Doping Agency to analyze Sochi samples “in the most sophisticated and efficient way possible.” It’s unclear what they might find, however, as Rodchenkov said he substituted tainted samples for clean ones.

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