Cyclist David Zabriskie was among four high-profile riders who asked not to be considered for the U.S. men's team that will compete at the London Olympics this summer, USA Cycling said.
In a short statement Saturday, the federation said that Zabriskie Â a native of Salt Lake City who graduated from Olympus High School George Hincapie, Levi Leipheimer and Christian Vande Velde each made the request on his own.
"USA Cycling will not speculate on the reasoning behind their requests and will not have further comment on this topic," it said, the day after announcing its five-man road team.
The federation said it issued the statement because of "numerous inquiries" about its choices.
The exclusion of the four riders has fueled speculation that they might be among the witnesses in the latest case of alleged doping against Lance Armstrong.
All are former Armstrong teammates on the U.S. Postal Service team.
But the U.S. Anti-Doping Agency has said it will not disclose the names of its witnesses in the Armstrong case, for fear of intimidation. It's possible that USA Cycling wanted to avoid the potential embarrassment of sending a rider to London, only to have his name surface later in a doping scandal.
Hincapie announced last week that he plans to retire at the end of the year after racing the Tour de France, while Leipheimer a Rowland Hall graduate told VeloNews that he did not expect to get a spot on the five-man Olympic road racing team because he's still recovering from injury.
Zabriskie, a seven-time national time trial champion who twice won medals at the world championships, could not be reached for comment.
Rising star Taylor Phinney will race the time trial instead of Zabriskie for the Americans in London, joining Timmy Duggan, Tyler Farrar, Chris Horner and Tejay van Garderen on the team.