Several track athletes tested positive for a steroid that until recently was undetectable and face suspensions that could bar them from the 2004 Athens Olympics, the U.S. Anti-Doping Agency (search) said Thursday.

USADA chief executive officer Terry Madden called it a widespread "conspiracy" involving chemists, coaches and athletes that was brought to the agency's attention by an anonymous tip.

He said the inquiry has expanded to other U.S. professional sports, but wouldn't give specifics. He also refused to give details about the athletes or say how many tested positive for the steroid, known as tetrahydrogestrinone (search), or THG.

"What we have uncovered appears to be intentional doping of the worst sort," Madden said in a statement before his conference call from USADA headquarters in Colorado Springs, Colo. "This is a far cry from athletes accidentally testing positive as a result of taking contaminated nutritional supplements."

Olympic athletes face drug tests at major competitions, as well as random testing between events. Their samples are divided in two and stored for future reference.

The athletes whose "A" samples revealed THG have been notified and will now have their "B" samples tested. If those are positive, a review process will begin. Appeals could last for months. Track athletes found to have used steroids would face two-year bans.

THG has a chemical structure similar to the banned anabolic steroids gestrinome (search) and trenbolone (search), Madden said. Though THG is not specifically named as a banned substance in world track, it would be considered a related substance outlawed under the sport's doping rules.

"This is a serious warning for cheaters," said Dick Pound, chairman of the World Anti-Doping Agency. "It shows that supposedly undetectable substances can be detected as new tests are developed."

Madden said the USADA received a call from a man in June claiming to be a track coach and naming athletes whom he claimed were using an undetectable steroid. He then sent the agency a used syringe containing some of the substance, Madden said.

The anonymous tipster, Madden said, identified the source of the THG as Victor Conte, founder of BALCO laboratory of Burlingame, Calif. The lab supplies nutritional guidance and supplements to athletes.

Conte did not respond Thursday to an e-mail inquiry from The Associated Press, and calls to BALCO went unanswered.

In an e-mail to the San Francisco Chronicle and San Jose Mercury News, Conte denied BALCO was the source of the substance.