WADA identified the group of hackers as Tsar Team (APT28), also known as Fancy Bear.
The www.fancybear.net website said it had information about a number of U.S. athletes, including tennis sisters Serena and Venus Williams as well as multiple gold medal-winning gymnast Simone Biles.
"WADA condemns these ongoing cyber-attacks that are being carried out in an attempt to undermine WADA and the global anti-doping system," said director general Olivier Niggli in a statement that did not name any athletes.
