Vonn's spokesman, Lewis Kay, wrote in an email that the four-time overall World Cup champion was "awaiting results from diagnostic testing for severe intestinal pain."
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Vonn's ski technician, Heinz Haemmerle, told The Associated Press that this isn't the first time the racer has gone to the hospital in Vail.
Haemmerle, who prepares Vonn's Head skis and has been on hand in Colorado awaiting her recovery, said that Vonn hasn't trained since going out in the second run of the season-opening giant slalom Oct. 27 in Soelden, Austria.
"She told me she feels bad and has pain all over her body and that her bones are hurting. ... She's been (to the hospital) two or three times. This is the first time she's stayed overnight," Haemmerle said in a telephone interview. "The coaches also don't know. First they told me we would train again Monday, then Wednesday, now the end of the week."
Vonn recently requested to compete in a men's downhill race, only to be rejected by the International Ski Federation. She was hoping to enter the men's race Nov. 24 at Lake Louise, Alberta. Had she been allowed to compete against the men, Vonn would have missed the two women's races in Aspen, Colo., because they take place the same weekend.
Vonn also skipped a slalom in Levi, Finland, last weekend, although Haemmerle said that was their plan even before the illness — to pick up more training for Aspen.
Haemmerle said that Vonn attended the Snow Ball in New York after Soelden, then went to Colorado.
"There's been no news, so we're kind of worried," Head racing director Rainer Salzgeber said from Austria. "It's not good."
Both Haemmerle and Salzgeber confirmed that Vonn had no broken bones or skiing injuries.