WADA says the group illegally gained access to confidential medical data
A Russian cyber espionage group has been condemned by the World Anti-Doping Agency for hacking into its database and stealing information about US Olympic stars.
WADA says the group, calling itself Fancy Bear, illegally gained access to confidential medical data, which they then leaked.
Among the sports stars whose details were posted online were tennis players Serena and Venus Williams and top gymnast Simone Biles.
The hackers claimed the records showed the three women had been doping, but the records appear to show that the substances they took had been cleared on medical grounds.
Olivier Niggli, director-general of WADA, said: "WADA condemns these ongoing cyber-attacks that are being carried out in an attempt to undermine WADA and the global anti-doping system.
"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."
The hack comes in the wake of a ban on a number of Russian athletes following the release of a report by investigator Richard McLaren, which found evidence of Russian state-sponsored doping.
The documents posted online say one of the athletes, multi-gold medal winner Biles, was found to have taken a drug commonly used to treat Attention Deficit Disorder (ADD), among other conditions.
Biles later tweeted that she suffers from Attention Deficit Hyperactivity Disorder (ADHD).
A statement from USA Gymnastics said 19-year-old Biles, who won four gold medals in Rio, had submitted and was given approval to use medication for therapeutic reasons.
She was provided with a Therapeutic Use Exemption (TUE), the paperwork necessary to allow an athlete to take a prescribed drug which is on the WADA prohibited list.
Steve Penny, president of USA Gymnastics, said: "Simone has filed the proper paperwork per USADA and WADA requirements, and there is no violation.
"The International Gymnastics Federation, the United States Olympic Committee and USADA have confirmed this. Simone and everyone at USA Gymnastics believe in the importance of a level playing field for all athletes."
Serena and Venus Williams, according to the leaked records, were also approved to use a prohibited substance.
All their records, underneath a heading that says "certificate of approval for therapeutic use", say: "The Athlete has received approval for the use of the prohibited substances(s) listed below under the conditions stipulated in this document."
An American female basketball player was also targeted.
Fancy Bear says on its website: "We are going to tell you how Olympic medals are won. We hacked World Anti-Doping Agency databases and we were shocked with what we saw.
"We will start with the US team which has disgraced its name by tainted victories. We will also disclose exclusive information about other national Olympic teams later."
Travis T Tygart, the CEO of the US Anti-Doping Agency (USADA), said: "It's unthinkable that in the Olympic movement, hackers would illegally obtain confidential medical information in an attempt to smear athletes to make it look as if they have done something wrong.
"The athletes haven't. In fact, 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."