Russian hackers — possibly the same group that compromised the Democratic National Committee’s computer servers — have made top American athletes their latest target.

Joining an intercontinental dispute over sports doping, the hackers penetrated the World Anti-Doping Agency’s athlete database and publicly revealed private medical information about three of the United States’ most famous athletes: Serena Williams, Venus Williams and Simone Biles.

The hackers published documents this week showing that Ms. Biles, who won four gold medals in gymnastics at the Rio Olympics last month, and the Williams sisters received medical exemptions to use banned drugs.

The antidoping agency confirmed the authenticity of the documents in a statement Tuesday, attributing the hack to Fancy Bear, a Russian cyberespionage group that forensics specialists have tied to breaches against government agencies, nonprofit organizations and corporations. That group is believed to be associated with G.R.U., the Russian military intelligence agency suspected of involvement in the recent theft of emails and documents from the D.N.C.