Russian hackers broke into a World Anti-Doping Agency database and posted confidential medical data of some American athletes online.
WADA said Tuesday the attack—which targeted some female members of the United States team which competed at the Rio de Janeiro Olympics—was carried out by a "Russian cyber espionage group" called Fancy Bears.
The hackers revealed records of "Therapeutic Use Exemptions" which allow athletes to use substances that are banned unless there is a verified medical need.
WADA previously warned of cyberattacks after investigators it appointed published reports into Russian state-sponsored doping.
"These criminal acts are greatly compromising the effort by the global anti-doping community to re-establish trust in Russia," World Anti-Doping Agency director general Olivier Niggli said in a statement.
WADA said it "extended its investigation with the relevant law enforcement authorities."
Last month, hackers obtained a database password for Russian runner Yuliya Stepanova, a whistleblower and key witness for the WADA investigations. She and her husband, a former official with the Russian national anti-doping agency, are now living at an undisclosed location in north America.
The International Olympic Committee said it "strongly condemns such methods which clearly aim at tarnishing the reputation of clean athletes."
"The IOC can confirm however that the athletes mentioned did not violate any anti-doping rules during the Olympic Games Rio 2016," the Olympic body said.
Those behind the breach have adopted the name "Fancy Bears," an apparently tongue-in-cheek reference to a collection of hackers which many security researchers have long associated with Russia.
In a statement posted to its website early Tuesday, the group proclaimed its allegiance to Anonymous, the loose-knit movement of online mischief-makers, and said it hacked WADA to show the world "how Olympic medals are won."
"We will start with the U.S. team which has disgraced its name by tainted victories," the group said, adding that more revelations about other teams were forthcoming.
Internet records suggest Fancy Bears' data dump has been in the works for at least two weeks; their website was registered on Sept. 1 and their Twitter account was created on Sept. 6.
Messages left with the group were not immediately returned.
Explore further: Russia sees 'nothing wrong' in athletes inhaling xenon