American anti-doping chiefs have revealed "several" US athletes have tested positive for a new anabolic steroid.
The positive A samples came from 350
doping tests at June's US championships and 100 later out-of-competition tests
The United States Anti-Doping Agency would not name the athletes who tested positive for tetrahydrogestrinone.
"What we have uncovered appears to be intentional doping of the worst sort," said USADA chief Terry Madden.
In what is described as the largest drug bust in athletics history, the USADA said a number of international athletes had also tested positive and there had also been positive samples for the stimulant modafinil.
The substance tetrahydrogestrinone was initially found during analysis of the contents of a syringe handed in, under anonymity, by a "top" athletics coach.
"This is a conspiracy involving chemists, coaches and certain athletes using what they developed to be undetectable designer steroids," said Madden.
"This is a very sophisticated designer steroid created by very sophisticated chemists.
"The athletes who did take part in this were very sure of themselves in being undetectable.
"Although the effects would last months, we believe the detection of the steroid in the body would only last a short period of time, three to seven days."
The International Association of Athletics Federations and the athletes involved have been notified of the USADA's findings, but no names are likely to be released until December.
All of the athletes with positive tests would be subject to a two-year ban and would be ineligible for the 2004 Olympics if their B samples are also positive.
World Anti-Doping Agency president Dick Pound welcomed the findings.
"We have to wait until the B samples are analysed and proper process completed," said Pound.
"But this is a serious warning for cheaters. It shows that supposedly undetectable substances can be detected as new tests are developed."