Tour de France winner Floyd Landis failed to show up for a one-day race in Denmark on Thursday, a day after missing a scheduled event in the Netherlands.
Danish organizers said they had arranged a contract and plane tickets for Landis and Phonak teammates to participate in the 1,200-meter Grand Prix Jyske Bank race in Silkeborg, 240 kilometers (149 miles) west of Copenhagen.
"He was not on the plane," organizer Jesper Tikoeb said. "We don't know where he is. It's pretty mysterious. We know nothing, really nothing at all."
He said his calls to the Phonak team went unanswered. Landis also failed to appear for the Acht van Chaam race in the Netherlands on Wednesday.
"We were told by the other two riders that he couldn't join because he was traveling with team manager (John) Lelangue to his doctor in Germany," event organizer Theo van der Westerlaken said. "That's all we know."
Landis did take part in a criterium race on Tuesday in Stiphout, Netherlands, winning the event.
Cycling Services, which booked teams for both Dutch events, said it has been trying without success to get in contact with Phonak for an explanation.
Cycling Services spokesman Marten van Steen said another Phonak rider, Robert Hunter, indicated that Landis had gone to see his doctor in Germany for his hip problem.
Landis said during the Tour de France that he was riding with an arthritic hip and would need replacement surgery. "I don't know any more than what I was told last night, that he (Landis) had trouble with his hip and needed to go to his doctor in Germany," another Phonak rider, Koos Moerenhout, said.
Contacted by The Associated Press, the Phonak press office referred calls to Lelangue. Calls to Lelangue's mobile phone went unanswered, and messages weren't immediately returned.
Landis won the Tour de France on Sunday, keeping the title in US hands for the eighth straight year after Lance Armstrong's record seven victories.
The International Cycling Union said on Wednesday that a cyclist failed a doping test during the tour. It said the name, team and nationality won't be released until the testing process is completed, including the analysis of a backup sample.
UCI did say the rider's team and national federation were notified, as well as national and world anti-doping authorities. USA Cycling has not been contacted by UCI, spokesman Andy Lee said on Wednesday.
US Anti-Doping Agency spokeswoman Carla O'Connell said that group had no comment.
On the eve of the Tour's start, nine riders, including pre-race favorites Jan Ullrich and Ivan Basso, were ousted, implicated in a Spanish doping investigation.