Four US athletes were confirmed to have given positive 'A' sample urine tests for the previously unknown steroid tetrahydrogestrinone (THG) together with Britain's leading sprinter Dwain Chambers. All five are now awaiting the results of 'B' tests.
Chambers tested positive for THG in August but on Wednesday he denied wilfully taking the drug. Under current rules he faces a two-year ban from the sport if found guilty.
US Track and Field (USATF) reacted by announcing lifetime bans and $100,000 fines for any American athletes found to have used anabolic steroids under a new 'Zero Tolerance' initiative.
The revelations, less than a year before the Athens Olympics, left the Games' central sport tarnished and discredited as officials scrambled to defend its credibility.
Athletics' global ruling body, the International Association of Athletics Federations (IAAF), said on Tuesday it would re-test samples taken at August's world championships in Paris to look for the previously undetectable drug.
France's Minister for Sport Jean-Francois Lamour said on Wednesday he had agreed with the IAAF that samples taken at the world championships would be re-tested for THG at a Paris laboratory and not sent to Los Angeles.
Details of how to detect THG, a steroid tweaked by chemists to evade detection under normal test conditions, have been distributed to 30 International Olympic Committee (IOC) accredited laboratories worldwide within days of the drug's discovery being made public.
One laboratory in Cologne, Germany, has already started re-testing hundreds of athletes' urine samples. Its head, Wilhelm Schaenzer, said on Wednesday: "We know what we're looking for and we know how to find it."
Scientists at the University of California, Los Angeles, uncovered THG after a tip-off from a coach in early June.
Both the World Anti-Doping Agency (WADA) and the IOC on Wednesday welcomed
The USATF, which had been criticised for dragging its feet over doping issues, justified its stringent new sanctions saying: "The USA Track and Field is at a crossroads.
"To the extent that we succeed by catching more cheaters, the short term pain will be significant. But there is no responsible choice other than to follow this course of action."
The USATF said the four U.S. athletes, none of whom it named, had given positive A sample tests for THG during re-testing of urine samples from the U.S. championships in June and during out-of-competition testing.
None were medallists at this year's world championships.
Chambers finished fourth in the 100 metres final in Paris but also anchored the British team to a silver medal in the 4x100 relay. The British team face being stripped of that medal if Chambers' positive test is confirmed by his 'B' sample.
A statement issued by Chambers' lawyers said he "will not accept nor tolerate any accusations or implications that this was a wilful or calculated attempt on his behalf to deceive the authorities.
"In his eight years in international athletics he has never been tempted to succumb to illegal methods of enhancing a performance."
Chambers' lawyers said the European 100 metres champion had been assured by his nutritionist Victor Conte that any supplements he had been given to take were within IAAF rules.
UK Athletics' chief executive David Moorcroft described the revelations as "depressing", confirming he had been informed of Chambers' positive test by the IAAF at the end of September.
He pointed out that under current rules the athlete was responsible for what he took. "The athlete has to carry the can for that," Moorcroft said.
"It always hurts when any British athlete is involved in anything like this.
"In the past we have had the shock of dealing with athletes who have been involved with, broadly speaking, contaminated supplements but this is a completely different kettle of fish."