Unknown attackers have hacked into Russian whistleblower Yuliya Stepanova`s World Anti-Doping Agency (WADA) records, the watchdog announced Saturday.
The runner, who was excluded from the Rio Olympics and lives in hiding in the United States, has been a key source of information on accusations that the Russian government organised doping on a massive scale.
WADA said that Stepanova`s password for her account with the agency had been "illegally obtained, which allowed a perpetrator to access her account".
Athletes have to detail their movements on the account so that dope testers know where they can be found.
WADA said its security monitoring had "noted that someone, other than Ms. Stepanova, had accessed her account.
"WADA immediately locked Ms. Stepanova`s account to prevent further access and notified her of the situation."
The agency said it was monitoring whether other athletes had been caught up in the phishing attack, where users are tricked into giving their personal details.
"The agency confirmed that some users had received illegitimate emails that look as though they come from WADA, which ask users to click on a link and enter their personal credentials," a WADA statement said.
"We continue monitoring the situation to determine whether or not users have acted (or act) on these emails to ensure that data remains secure," it added.
"Unfortunately, like many organisations, WADA is not immune to attempted cyber-attacks... WADA is in contact with the relevant law enforcement authorities."
Separately, International Olympic Committee spokesman Marc Adams said the body was also undergoing "particularly intense" attacks from hackers during the Games.
"It`s not unusual to be subject to attempts to break our security... we come under regular attack from people trying to break into our security," he said.