Story highlights IOC finds drug offenses in 2008 retests

Plans to retest London 2012 athletes

Samples are kept for 10 years

Aims to clean up sport for Rio Games

(CNN) Thirty-one athletes hoping to compete at August's Olympics tested positive for banned substances at the 2008 Beijing Games and could now be ruled out of Rio, the International Olympic Committee says.

The IOC didn't reveal where the athletes came from or their names, but said 12 countries and six different sports were involved.

The ruling body said its executive board "agreed unanimously to initiate proceedings immediately, with the 12 (national Olympic committees) concerned informed in the coming days."

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"All those athletes infringing from anti-doping rules will be banned from competing at the Olympic Games Rio 2016," it added.

Tuesday's revelation only thickens the black cloud surrounding drugs and the Olympics, much of it involving Russia -- whose track-and-field athletes were banned from competing from Rio this year due to regular doping in the past.