The World Anti-Doping Agency said late Wednesday that a group of Russian "cyber-hackers" called Fancy Bears had leaked another batch of confidential athletes' information from its database.
WADA said that similar to the leak that the agency announced on Tuesday, the group released confidential data of 25 athletes from eight countries "into the public domain."
It said the targeted athletes included 10 from the United States, five from Germany, five from Britain, and one each from Czech Republic, Denmark, Poland, Romania and Russia. It did not identify the athletes.
On Tuesday, confidential medical data of gold medal-winning gymnast Simone Biles, seven-time Grand Slam champion Venus Williams and other female US Olympians was hacked and posted online.
They revealed records of "Therapeutic Use Exemptions" (TUEs), which allow athletes to use otherwise-banned substances because of a verified medical need.
WADA said Wednesday that the Russian group had again illegally gained access to its Anti-Doping Administration and Management System, or "ADAMS," and said it included confidential medical data such as TUEs.
"WADA is very mindful that this criminal attack, which to date has recklessly exposed personal data of 29 athletes, will be very distressing for the athletes that have been targeted and cause apprehension for all athletes that were involved in the Rio 2016 Olympic Games," WADA director general Olivier Niggli said in a statement.
"To those athletes that have been impacted, we regret that criminals have attempted to smear your reputations in this way; and, assure you that we are receiving intelligence and advice from the highest level law enforcement and IT security agencies that we are putting into action."
Niggli said WADA had "no doubt that these ongoing attacks are being carried out in retaliation against the agency, and the global anti-doping system," because of independent investigations that exposed state-sponsored doping in Russia.
Williams, who won a silver medal in mixed doubles at the Rio Olympics last month, issued a statement via her agent saying she was granted TUEs "when serious medical conditions have occurred," and those exemptions were "reviewed by an anonymous, independent group of doctors, and approved for legitimate medical reasons."
In a statement, USA Gymnastics said Biles - who won four gold and one silver medal in Rio last month - was approved for an exemption and had not broken any rules.
Last month, hackers obtained a database password for Russian runner Yuliya Stepanova, a whistleblower and key witness for the WADA investigations. She and her husband, a former official with the Russian national anti-doping agency, are now living at an undisclosed location in North America.
The International Olympic Committee said after Tuesday's WADA statement that it "strongly condemns such methods which clearly aim at tarnishing the reputation of clean athletes."
"The IOC can confirm however that the athletes mentioned did not violate any anti-doping rules during the Olympic Games Rio 2016," the Olympic body said.
The name "Fancy Bears" appears to be a tongue-in-cheek reference to a collection of hackers that many security researchers have long associated with Russia.
In a statement posted to its website early Tuesday, the group proclaimed its allegiance to Anonymous, the loose-knit movement of online mischief-makers, and said it hacked WADA to show the world "how Olympic medals are won."
"We will start with the US team which has disgraced its name by tainted victories," the group said, warning that more revelations about other teams would be forthcoming.