The World Anti-Doping Agency said Tuesday that Russian hackers broke into its database, stealing medical data of Olympic stars including gold medal gymnast Simone Biles and tennis player Venus Williams.

The cyber criminal group known as "Tsar Team" and "Fancy Bear" publicly released some of the stolen data and threatened to leak more in the future. The same group is thought to have been behind the Democratic National Committee hack in June -- which released sensitive political strategy and led to the resignation of the committee chairman.

On the website, which CNN cannot confirm as legitimate, Fancy Bears offered a message which said in part, "Greetings citizens of the world. Allow us to introduce ourselves. We are Fancy Bears' international hack team. We stand for fair play and clean sport."

The hackers said, "We will also disclose exclusive information about other national Olympic teams later. Wait for sensational proof of famous athletes taking doping substances any time soon."

USA Gymnastics said in a statement that among the files breached were the drug testing results for Biles, who won four gold medals in the Rio games last month. The group said that Biles "was approved for a therapeutic-use exemption" of certain drugs and "has not broken any drug-testing regulations, including at the Olympic Games in Rio."

Biles tweeted that she has ADHD and said that she believes in clean sport and has always followed the rules.

"Having ADHD, and taking medicine for it is nothing to be ashamed of nothing that I'm afraid to let people know," she wrote.

She was supported by USA Gymnastics.

"Simone has filed the proper paperwork per USADA (U.S. Anti-Doping Agency) and WADA requirements, and there is no violation," said Steve Penny, president of USA Gymnastics.

Venus Williams issued a statement acknowledging her drug test results had been hacked and said that she had also been granted "therapeutic use exemptions," although she doesn't cite the reason for the exemption.

"I am one of the strongest supporters of maintaining the highest level of integrity in competitive sport and I have been highly disciplined in following the guidelines," she wrote.

The drug testing results for star basketball player Elena Delle Donne were also revealed in the hack. "I'd like to thank the hackers for making the world aware that I legally take a prescription for a condition I've been diagnosed with, which WADA granted me an exemption for. Thanks, guys!" she posted.

USADA CEO Travis Tygart said the athletes are being wrongly smeared.

"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," he said.

WADA's database was accessed through an account created by the International Olympic Committee for the Rio games, WADA said.

In a statement, WADA claims a cyber-espionage group obtained the information through phishing of email accounts, which allowed hackers to obtain passwords to WADA's database, after receiving fake emails from an address known to the recipient, asking for sensitive information.

The Russian government denied any involvement in the hack, according to a statement from Russian presidential spokesman Dmitry Peskov, which was reported by the state run media agency TASS.

"We can say without a hesitation any involvement in such actions on the part of official Moscow, the Russian government or any Russian secret services is strictly out of the question. It's simply ruled out," Peskov said.

This is not the first time WADA was targeted. The anti-doping agency database account of whistleblower Yuliya Stepanova was hacked in August. The runner helped expose the scale of Russian doping problems last year.

"WADA condemns these ongoing cyber-attacks that are being carried out in an attempt to undermine WADA and the global anti-doping system," said Olivier Niggli, the agency's director general.

He added that the hack is "greatly compromising the effort by the global anti-doping community to re-establish trust in Russia."

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WADA is not popular in Russia. It had recommended banning all Russian athletes from the Rio 2016 games, after an independent report said the country operated a state-sponsored doping program during the 2014 Sochi Winter Games. Russian officials and athletes likened the move to Cold War era conflicts.

The International Olympic Committee didn't issue a blanket ban on Russian athletes in Rio, leaving the decision on competitors' eligibility up to their respective sporting federations.

Many Russian track and field athletes and were banned from competing by their sports' governing bodies after the International Olympic Committee set strict anti-doping standards.