Top track coach Alberto Salazar, banned for four years following a doping probe, used athletes as "guinea pigs", the US Anti-Doping Agency's chief Travis Tygart told German broadcaster ZDF on Wednesday.
The World Athletics Championships, currently taking place in Doha, has been rocked by the scandal and Salazar has been stripped of his accreditation for the event.
Salazar has denied ever doping his athletes and says he will appeal.
Tygart told ZDF that athletes in Salazar's Nike-backed Oregon Project (NOP) training group were kept in the dark about the substances they were given, including whether they were illegal.
Tygart said the methods used by Salazar and doctors working with NOP were "simply unacceptable".
"The athletes really had no idea what they were being given, the dosages, whether the methods were prohibited or not. They were simply sent to a doctor and told, 'you've got to listen to the doctor'."
Tygart said he hoped Nike, which has long sponsored Salazar, would review their cooperation with the coach.
"I hope Nike sees this as a wake-up call," Tygart said,
Nike has backed Salazar in his decision to appeal the ban, adding: "Nike does not condone the use of banned substances in any manner."
Two members of the Oregon Project have won gold medals at the world championships in Doha, including American Donavan Brazier, who powered to victory in Tuesday's 800 metres final in a championship record time.
However, none of the athletes taking part at the world championships linked to Salazar's group have been found guilty of doping offences, and none were implicated in USADA's 134-page summary of the case.
Alongside Salazar, Jeffrey Brown, a Texas endocrinologist who treated athletes at the Oregon training hub in Portland, was also suspended for four years.