A diplomatic row breaks out after an American coach describes the performance of a Chinese swimmer as suspicious and disturbing.
Ye Shiwen, 16, has denied doping and says her results come from hard work and training.
She smashed a world record in Saturday's women's 400 metre medley - swimming the final 50 metres of the race faster than the men's champion, Ryan Lochte.
John Leonard, the executive director of the World Swimming Coaches Association, told the Guardian: "We want to be very careful about calling it doping.
"The one thing I will say is that history in our sport will tell you that every time we see something, and I will put quotation marks around this, 'unbelievable', history shows us that it turns out later on there was doping involved.
"That last 100m was reminiscent of some old East German swimmers, for people who have been around a while. It was reminiscent of 400m individual medley by a young Irish woman in Atlanta."
That was a reference to Irish swimmer Michelle Smith who was banned for four years in 1998, two years after Atlanta, after testing positive for androstenedione.
Leonard said: "She looks like superwoman. Any time someone has looked like superwoman in the history of our sport they have later been found guilty of doping".
Ye Shiwen took five seconds off her personal best and more than a second off the world record in the 400m individual medley.
Her swim was described as "insanely fast" by previous world record holder Steph Rice.
Ye herself insists the team have nothing to hide, saying: "The Chinese team keep very firmly to the anti-doping policies, so there is absolutely no problem."
IOC medical commission chairman Arne Ljungqvist, a veteran anti-doping competitor, did not express any concerns when asked if he had any suspicions regarding the swimmers in London.
Ljungqvist said: "I am pretty experienced in this matter, as you know, and have been at the Games since a long time and within anti doping for 40 years.
"You ask me specifically about this particular swimming. I say no, I have not personally any reason other than to applaud what has happened, until I have further facts, if so."
Users of China's popular microblogging site Weibo - the country's equivalent to Twitter - quickly leapt to the swimmer's defence, accusing the US and UK of "jealousy".
"It's not classy at all to say that record-breakers have taken drugs. It's just jealousy," wrote one user.
Another post said: "She's just a child. Don't be so beastly to her."
But others expressed their doubts.
"Maybe the Chinese have discovered some sort of new drug, for how could she suddenly have become this strong?" said one blogger.
Over a million Weibo users have so far posted on the subject.