"Observing the physiques of some successful athletes who have not tested positive suggests that they are using something that is not being detected."
Scientists are working around the clock at a drug-testing lab in Beijing to test for the methods and substances, such as stimulants, steroids, hormones and narcotics, banned by the World Anti-Doping Agency.
But, UCLA's Butch emphasized, athletes are increasingly seeking out short-acting drugs that clear the body very quickly. By monitoring the impact of those drugs on other molecules in the body, scientists have been able to catch cheaters.
Scientists determined last week, for example, that Spanish cyclist Maria Isabel Moreno had used the banned hormone erythropoietin. But because the drug exits the body rapidly it's often difficult for scientists to detect.
Other drugs that are eliminated quickly, Butch said, are composed of insulins and growth hormones. Gene doping is another method that might soon catch on.
Researchers say gene doping presents numerous health risks, such as leukemia and, even, death. But the anti-doping community believes athletes are eyeing it because of its ability to fly under the radar of doping tests.
In addition to the scientific hurdles associated with catching Olympic dopers, some experts say a monitoring structure that relies heavily on national anti-doping authorities also allows cheaters to evade detection.
"The Olympic testing process for performance enhancing drugs is a farce," Jamie Metzl, an executive vice president of the Asia Society, who has written extensively about international anti-doping efforts, told ABCNews.com in an e-mail. "[The process] cannot be successful because the tests are inadequate and often administered by national sports authorities with far stronger incentives to win than crack down on wrongdoers."