United States Anti-Doping Agency (Usada) chief executive Travis Tygart has labelled the Fancy Bears "con artists" after the hacker group accused US sports officials of covering up doping.
The Russian group has leaked medical records of several global sports stars, having stolen them from the World Anti-Doping Agency's (Wada) database, and on Thursday released email exchanges it claimed revealed that "Usada covers up many athletes using prohibited substances".
Tygart has hit back, dismissing the group's latest release as "desperate" and "malicious".
"This is just another desperate attempt to distract from the real issue of (Russian) state-sponsored doping," Tygart told BBC Sport.
He said the Fancy Bears were trying to "smear the reputations of athletes and organisations from around the world who choose to operate with integrity and abide by the rules".
"They are con artists," he continued. "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."
Thursday’s latest releases were emails which detailed more athletes who were given therapeutic use exemptions (TUEs), which allow athletes to take banned substances for verified medical needs.
Usada’s 2015 annual reports said that 402 TUE applications by athletes were granted.