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.

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 & 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.

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.