United States Anti-Doping Agency chief executive Travis Tygart has labelled the Russian cyber espionage group as "con artists" after the hackers levelled "cover ups" allegation at the USADA.
The group, identified as Fancy Bears, has posted data about athletes such as U.S. gymnast Simone Biles, basketball player Elena Delle Donne and tennis players Serena and Venus Williams.
The World Anti-Doping Agency (WADA) said the hackers gained access to its anti-doping administration and management system (ADAMS) via an account for the Rio Games, which has since been secured and investigated by a premier security and forensic consulting firm.
According to WADA, the account included confidential medical data such as Therapeutic Use Exemptions, which are issued by sports federations and national anti-doping organizations to allow athletes to take certain substances.
"They are con artists. We are confident that people will see this for what it is: a malicious and illegal invasion of athlete privacy followed by a baseless smear campaign," Tygart told the BBC.
"This is just another desperate attempt to distract from the real issue of (Russia's) state-sponsored doping."
Since the hacking group first released confidential medical data on Sept. 13, it has released five more batches involving athletes from countries including the U.S., Germany, Britain, Poland and Denmark who competed at the Rio Olympics.
Tygart added that they "smear the reputations of athletes and organisations from around the world who choose to operate with integrity and abide by the rules".
(Reporting by Ian Rodricks in Bengaluru; editing by Amlan Chakraborty)