The World Anti-Doping Agency said on Tuesday that hackers stole confidential medical information about U.S. Olympic athletes and published it on the internet, blaming a Russian group for the attack.
The U.S. government is investigating the case because there is evidence that the hackers are linked to the Russian government, though details are still sketchy, according to two sources familiar with the probe who were not authorized to publicly discuss the matter.
Russian news agencies quoted Kremlin spokesman Dmitry Peskov as saying any possible Russian government or secret service participation in the hacking was out of the question.
An FBI representative said she had no immediate comment on the release of the medical information, which prompted gymnast Simone Biles to disclose that she has an attention deficit hyperactivity disorder, or ADHD.
WADA issued a statement attributing the attack to Tsar Team, a hacking group widely known as APT28 and Fancy Bear by U.S. cyber-security researchers.
Fancy Bear is one of two hacking groups accused in June of hacking the Democratic National Committee's computer network. CrowdStrike, a firm hired by the DNC to respond to those attacks, said in June that Fancy Bear was probably working on behalf of the Russian military.
WADA said that law enforcement had told it the attacks originated in Russia. WADA spokeswoman Maggie Durand declined to elaborate or say how the operation had been uncovered.
"WADA condemns these ongoing cyber-attacks that are being carried out in an attempt to undermine WADA and the global anti-doping system," said Director General Olivier Niggli in a statement.
WADA said it believed the hackers gained access to its anti-doping administration and management system (ADAMS) via an IOC-created account for the Rio Games.
The doping agency made the accusations as a website, www.fancybear.net, posted what appeared to be data about four U.S. athletes: Simone Biles, Elena Delle Donne, Serena Williams and Venus Williams.
That site, which internet registration records said was created on September 1, said it planned disclosing information about athletes from other nations in the future.
On Tuesday it released documents known as Therapeutic Use Exemptions, or TUEs, which are issued by sports federations and national anti-doping organizations to allow athletes to take certain substances.
The leak of a TUE with information about Biles prompted her to disclose on Twitter that she has ADHD.
"I have ADHD and I have taken medicine for it since I was a kid," Biles said. "Please know, I believe in clean sport, have always followed the rules, and will continue to do so as fair play is critical to sport and is very important to me."
The International Olympic Committee (IOC), U.S. Anti-Doping Agency (USADA), International Tennis Federation (ITF) and USA Gymnastics all issued statements saying that athletes whose data had been released had done nothing wrong.
The IOC condemned the leak as an attempt to tarnish 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 group said in a statement.
"In each of the situations, the athlete has done everything right in adhering to the global rules for obtaining permission to use a needed medication," Travis Tygart, chief executive of USADA, said in a statement.
"The cyber-bullying of innocent athletes being engaged by these hackers is cowardly and despicable."
ITF president David Haggerty said all TUEs handed out to tennis players were done so in accordance with WADA rules.
USA Gymnastics said that Biles was approved for a TUE exemption and had not broken any rules.
WADA's chief, who apologised for the hack, said that it was "greatly compromising the effort by the global anti-doping community to re-establish trust in Russia", following release of the McLaren Investigation Report.
The independent McLaren report charged that Russians had swapped positive doping samples for clean ones during the Sochi winter Games, with the support of the Russian secret service.
WADA revealed last month that Russian whistleblower Yulia Stepanova's electronic account had been illegally accessed.
Stepanova, who is in hiding in North America, helped reveal the biggest state-backed doping programme in Russia and was forced to flee the country with her husband for fear of her life.
(Additional reporting by Karolos Grohmann in Athens, Martyn Herman in London, Amy Tennery in New York, Mark Hosenball and John Walcott in Washington DC; editing by Ken Ferris)