American sprinters Tim Montgomery and Chryste Gaines have both been accused of doping violations.
Montgomery and Gaines both received letters from the USADA
World 100m record holder Montgomery and Olympic relay medallist Gaines received letters from the US Anti-Doping Agency.
The charges relate to the investigation into the Balco laboratory, which has been linked with new steroid THG.
Lawyer Cristina Arguedas said: "Tim Montgomery has not done anything wrong and we intend to fight any attempt to prevent him running in the Olympics."
Arguedas met USADA representatives late last week and was shown documents concerning Montgomery that were acquired from the Balco investigation.
She added: "The evidence we have been shown by USADA that we are still reviewing is inconclusive and internally inconsistent.
"There is also no foundational evidence to prove where it comes from and what it means."
"It is fundamentally unfair to take away an athlete's reputation, his work and his dream based on meagre information, flimsy documents and a flawed process."
USADA's legal affairs director Travis Tygart hopes to complete a review of suspected competitors by the 9-18 July track and field trials for the Olympic team.
Gaines had already been warned by USADA for testing postive for the stimulant modafinil at the US Championships in June 2003.
At the time modafinil was considered a minor stimulant that warranted only a disqualification.
Along with hurdlers Sandra Glover and Eric Thomas, she was stripped of her result in that competition - fifth in the 100m - but not barred from any other events.
This year, however, modafinil has been classified as a serious stimulant on the World Anti-Doping Agency banned list and carries a two-year ban.
Sprint champion Kelli White was given a two-year suspension after admitting taking modafinil as well as undetectable steroids and EPO.
White and Gaines are coached by Remi Korchemny, one of four men charged in February with distributing performance-enhancing drugs.
Three other Americans, shot putter Kevin Toth and hammer throwers John McEwen and Melissa Price have been banned after testing positive for THG.