Transgender hurdler Telfer ruled out of US trials
Telfer, who was due to compete in the opening heats of the women's 400m hurdles on Friday, was unable to provide evidence that she met testosterone thresholds laid out by World Athletics in 2019.
The World Athletics standards on transgender athletes apply to all events.
They are separate to the rules that apply to women athletes with unusually high levels of testosterone, who are prohibited from competing in distances between 400m and a mile unless they undertake treatment to lower their testosterone levels.
"USATF strongly supports inclusivity and providing a clear path to participation in the sport for all, while also maintaining competitive fairness," the federation said in a statement.
"If CeCe meets the conditions for transgender athlete participation in the future, we wholeheartedly back her participation in international events as a member of Team USATF."