Russian cybercriminal group ‘Tsar Team,’ or ‘Fancy Bear,’ hacked the Anti-Doping Administration and Management System of World Anti-Doping Agency and leaked confidential medical records of athletes who competed in the 2016 Summer Olympic Games in Rio de Janeiro, WADA confirmed Monday.

The hack most likely stemmed from a successful spearphishing attack, WADA reported.

The hackers used an International Olympic Committee-created account for the games and gained access to athlete data that included medical information like Therapeutic Use Exemptions, which allow athletes with medical needs to use certain banned substances for treatment.

Data from four-time gold medalist, gymnast Simone Biles and sisters Serena and Venus Williams were included in the in the leak. WADA officials said, law enforcement authorities have confirmed the attacks stem from Russia. The group threatened to release more data from other countries in the future.

“WADA condemns these ongoing cyber-attacks that are being carried out in an attempt to undermine WADA and the global anti-doping system,” Olivier Niggli, WADA’s director general, said in a statement.

“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.

The attack is purportedly a response to the banning of Russia’s track and field team over an alleged state-supported doping program. All athletes were also barred from the Paralympics, according to BBC.

The Russian government has denied any involvement in the WADA attack, BBC reported.