The World Anti-Doping Agency (WADA) has confirmed that a Russian cyber espionage group operator by the name of Tsar Team (APT28), also known as Fancy Bear, illegally gained access to its Anti-Doping Administration and Management System (ADAMS) database via an International Olympic Committee (IOC)-created account for the Rio 2016 Games.

The hacker group leaked confidential medical files of star US Olympic athletes, including tennis players Venus and Serena Williams and teenage gymnast Simone Biles.

Fancy Bears claimed responsibility for the hack of a WADA database.

After the leak, Simone Biles said she had long been taking medicine for ADHD (Attention Deficit Hyperactivity Disorder).

The hacker group had accused her of taking an “illicit psycho-stimulant”, but Simone Biles said she had “always followed the rules”.

The Rio Olympics quadruple gold medalist had obtained the necessary permission to take prescription medicine on the WADA banned drugs list, USA Gymnastics said in a statement.

In a statement on the agency’s official website, WADA Director General Olivier Niggli said: “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.”

“We are reaching out to stakeholders, such as the IOC, IFs and NADOs, regarding the specific athletes impacted,” he continued.

WADA said that the cyber attacks were an attempt to undermine the global anti-doping system.

Russian government spokesman Dmitry Peskov said it was “out of the question” that the Kremlin or secret services were involved in the hacking, Russian news agencies reported.

The hackers accessed records detailing “Therapeutic Use Exemptions” (TUEs), which allow the use of banned substances due to athletes’ verified medical needs.

“By virtue of the TUE, Biles has not broken any drug-testing regulations, including at the Olympic Games in Rio,” USA Gymnastics said.

Fancy Bears said TUEs amount to “licenses for doping”.

The leaked documents allege that Serena Williams was granted permission to use drugs commonly used to treat muscle injuries, such as anti-inflammatories, while Simone Biles is said to use Ritalin – a treatment for her ADHD.

Former – Australian Sports Anti-Doping Authority head Richard Ings said: “Nothing I see here gives me cause for alarm,” adding it looked “totally normal”.

“The issue here is privacy breach.”

Russia’s track and field team were banned from the Rio Olympics over an alleged state-backed doping program. All Russian athletes are barred from the ongoing Paralympics.

“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,” Olivier Niggli said.

US Anti-Doping Agency chief Travis Tygart called the hack “cowardly and despicable”.

“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.

The US Olympic Committee has had “zero adverse findings from the Rio Olympic Games that weren’t 100% within the medical guidelines set forth by anti-doping authorities,” spokesman Patrick Sandusky said.

Earlier this month, Olivier Niggli said WADA was experiencing almost daily cyber attacks originating from Russia.

Fancy Bears has pledged to release confidential records from other national Olympic teams.