Russian hackers released private medical information of top U.S. athletes, the World Anti-Doping Agency confirmed Tuesday. Hackers compromised the WADA’s database to obtain sensitive information on Olympic gold medalist Simone Biles, tennis stars Serena and Venus Williams and others.

The hackers published information which showed that Biles and the Williams sisters obtained permission to use banned substances for their medical conditions.

The anti-doping agency said the hacking collective Tsar Team, also known as Fancy Bear, was behind the breach. It said the group illegally gained access to WADA’s Anti-Doping Administration and Management System (ADAMS) database using an account created by the International Olympic Committee for the Rio Olympics 2016.

“WADA deeply regrets this situation and is very conscious of the threat that it represents to athletes whose confidential information has been divulged through this criminal act,” WADA's Director General Olivier Niggli said in a statement.

“WADA has been informed by law enforcement authorities that these attacks are originating out of Russia. Let it be known that these criminal acts are greatly compromising the effort by the global anti-doping community to re-establish trust in Russia further to the outcomes of the Agency’s independent McLaren Investigation Report,” he added.

USA Gymnastics also released a statement that confirmed that Biles had filed the necessary paperwork in order for her to take the medications on WADA’s Prohibited Drugs List for her Attention Deficit Hyperactivity Disorder.

The hacking collective reportedly vowed to release confidential data from other national Olympic teams too.

The same hacking collective was believed to be behind the breach of the Democratic National Committee’s servers, which resulted in the leak of the DNC’s sensitive political strategies and the resignation of the committee’s chair.