SAN FRANCISCO -- Its chemical components are similar to most banned steroids but with an insidious twist: This drug was synthesized so craftily it is undetectable by the standard test given to athletes.

The newly discovered designer steroid even foiled the skilled doping detectives who hunt for such drugs in urine samples, said Dr. Don Catlin of the Olympic Analytical Laboratory at UCLA.

What he and other scientists aren't so sure of is whether tetrahydrogestrinone -- THG -- was purposely designed to evade detection or if its creator got lucky.

"It could have been just a lucky shot,'' Catlin said. "But then I also tend not to underestimate the people who do this.''

THG is at the heart of what one U.S. anti-doping official called a widespread "conspiracy'' involving chemists, coaches and athletes that was brought to the agency's attention by an anonymous tip.

Already, Europe's fastest man -- 100-meter champion Dwain Chambers of Britain -- has admitted taking THG. Other athletes -- including sluggers Barry Bonds and Jason Giambi and boxer Shane Mosley -- have been subpoenaed to testify before a grand jury investigating the nutritional supplement company at the center of the unfolding case.

Because of the scandal, USA Track and Field, the sport's national governing body, proposed tougher drug rules Wednesday that could include a lifetime ban for a first steroid offense. The organization also disclosed that four of its athletes tested positive for THG, and they could be barred from the 2004 Olympics.

The U.S. Anti-Doping Agency, which monitors drug use by athletes in Olympic sports, turned to Catlin when it received a syringe of the stuff from an unidentified track coach this summer. After weeks of extensive tests, Catlin and his colleagues identified the substance as a "new chemical entity'' with all the hallmarks of an anabolic steroid.

Anabolic steroids are synthetic versions of the male hormone testosterone. Doctors prescribe them to AIDS patients and other disease-stricken people who lose muscle mass. Athletes use them illegally as chemical shortcuts to bulk up, build endurance and recover better from training.

They can have dangerous side effects, including liver damage, heart disease, anxiety and rage.

There are at least a hundred known anabolic steroids, and professional doping sleuths such as Catlin have amassed a library of chemical fingerprints of these illegal substances that they match against athletes' urine samples.

To find steroid traces in urine, scientists use gas chromatography and mass spectrometry testing. The testing involves drying the sample, adding chemicals and then heating it. But THG disintegrates during this process and goes undetected, Catlin said.

Catlin, however, said he has developed a new process that keeps the essential signature of THG from disintegrating. He said he is making his recipe available to international doping detection agencies.

The burgeoning scandal is the latest example of the cat-and-mouse game between makers of illicit drugs and the anti-doping officials who must constantly readjust their tests to detect ever-evolving substances.

Exactly who developed THG is unclear and is under investigation.

Victor Conte, owner of the Bay Area Laboratory Co-Operative recently raided by federal officials, has denied being the supplier of THG. He has not been charged with a crime, and his lawyers deny he has committed any wrongdoing.

Furthermore, Conte has said there is no proof THG is a steroid by the scientific definition. Catlin, however, said he is convinced it is.

Federal law makes possession of anabolic steroids illegal without a doctor's prescription. But some lawyers contend THG may not fit the legal definition of a steroid.

"It's apparently a newly created substance,'' said New York defense attorney Rick Collins, a former prosecutor who has defended clients accused of illegal steroid possession. "At this point we have no evidence, only speculation.''

Collins said a stronger criminal case could be made that THG is an unapproved drug that violates Food and Drug Administration regulations for the marketing of pharmaceuticals.

Either way, sports governing bodies worldwide are moving to crack down on THG use. The NFL has said the league might retest its samples for THG. Swimming's world governing body said it would consider retesting drug samples from its world championships this summer.

Anti-doping scientists said Catlin's discovery may confirm what they have suspected for years: that there is a robust underground trade in performance-enhancers created specifically to evade detection.

"This is junior chemistry compared to what's coming,'' warned Dr. Gary Wadler, a member of the World Anti-Doping Agency.