Olympic time-trial champion Tyler Hamilton (search) of the United States is being investigated for possible blood doping and could be stripped of his gold medal.

Tests at the Athens Olympics (search) on Aug. 19 and at the Spanish Vuelta (search) on Sept. 18 showed evidence of blood from another person, cycling's governing body said, according to a spokesman for Hamilton's team, Phonak. Follow-up tests were scheduled for later Tuesday.

If found guilty of a violation at the Olympics, Hamilton would be stripped of his gold. Three athletes had gold medals revoked for doping during the Aug. 13-29 games.

Spokesman Georges Luedinger said Hamilton denied having a transfusion — which can boost an athlete's performance by increasing the amount of oxygen-transporting red blood cells in his or her system.

"Tyler told us he did nothing," Luedinger said. Phonak planned a press conference in Zurich later Tuesday.

Hamilton would be the first American investigated for doping at the Athens Olympics. A record 24 athletes were caught for drug-test violations at the games.

Asked about Hamilton's reported positive test at the Olympics, IOC medical director Patrick Schamasch said, "For the moment, I can't confirm or deny anything."

IOC spokeswoman Giselle Davies said, "As with all doping procedures, while a process is underway, we can't go into details."

USA Cycling CEO Gerard Bisceglia said the IOC had not said anything to his organization regarding Hamilton's medal. He said he was waiting to see the results of the tests Hamilton was to take Tuesday.

"We're not in a position to take a position. We hope for the best with this, as we do with any athlete," Bisceglia said.

The U.S. Olympic Committee said it had no comment.

Hamilton's gold was one of four medals won by American cyclists at the Athens Games — the team's best showing since winning nine in Los Angeles in 1984, at an Olympiad boycotted by the Eastern Bloc.

Hamilton's mother said from her home in Marblehead, Mass., that the family learned of the tests Tuesday morning.

"This comes as a total shock to us," Lorna Hamilton said.

Hamilton was the only American cyclist to win a gold medal at the Athens Games, and he called that victory "the highlight of my career, by far."

If Hamilton is disqualified, the gold medal would go to Russia's Viatcheslav Ekimov, with American Bobby Julich moving up to silver and Australia's Michael Rogers to the bronze.

He left the Tour de France midway through the 13 stages because of a back injury, and was fourth in the 2003 Tour.

Hamilton pulled out of the Vuelta cycling tour after the 12th round on Sept. 16, citing stomach problems.