The International Olympic Committee announced yesterday it was dropping drug charges against U.S. cyclist Tyler Hamilton despite a "clearly positive" blood test at the Athens Games, saying an unfortunate human error in the laboratory made prosecuting the case impossible.

The decision means Hamilton, who became the first American in 20 years to win an Olympic road race, will keep the time trial gold medal he won on Aug. 18, but the achievement has been severely clouded by the IOC's assertion that he tested positive for blood doping, which means receiving banned blood transfusions to increase endurance.

Hamilton, who has vehemently maintained his innocence, still faces a possible two-year ban from the sport from the International Cycling Union (UCI) because of a second positive test in September during the Spanish Vuelta.

A confirmation test Tuesday backed up the Sept. 11 positive, Hamilton's team professional team Phonak announced yesterday.

The strange turn in the IOC case provided something of an embarrassment for the organization, which has taken a get-tough approach to drugs in recent years.

"It's not always like black and white, but to the expert panel [that examined Hamilton's test results] it was clearly black," IOC medical chief Arne Ljungqvist said, declining to comment on Hamilton's guilt or innocence. "It's up to anyone to make his own opinion on that. Obviously, the story for the athlete I believe is not finished."

In a statement before Ljungqvist discussed the case with reporters via conference call, Hamilton said: "I am sure that the gold medal that I worked so hard for will stay in my hands. I guarantee that I represented the United States as an honest, clean and proud athlete."

Ljungqvist said the testing lab in Athens erroneously froze the portion of Hamilton's blood sample -- what is known as the B sample -- reserved for confirmation testing, thereby destroying the cells that indicated the positive result. Ljungqvist said that without a confirmation test on the B sample that validates the A sample, the IOC's bylaws prohibit it from proceeding with a doping case.

"The rules are very clear," Ljungqvist said. "Legally, such a case would be deemed 'negative' because the B did not confirm the A."

Ljungqvist said that samples of urine, blood plasma and blood serum are typically frozen, which could explain the mistake, but that Hamilton's sample was a full blood sample and should not have been preserved in that manner. He said the World Anti-Doping Agency would have to decide whether to take action against the Athens lab.

"It's human error," he said. "It should not have been done with full blood. . . . The blood sample was unfortunately destroyed and couldn't be analyzed."

Hamilton's A sample was deemed by a group of independent experts to be conclusively positive on Sept. 16 after a lengthy review necessitated in part because the Athens lab director initially labeled it only a "suspicious" sample, Ljungqvist said.

Typically, the IOC announces positive tests at the Olympics within days of the competition.

The confusion over the result and the handling of it likely was due to the newness of the test, which was introduced just before the Athens Games and used to produce 385 samples. A group of scientists in Australia finalized a test last year modeled after the common paternity test that could detect the presence of foreign cells in the blood.

Previously, the IOC was unable to detect blood doping, which Ljungqvist speculated has been on the rise since the IOC began testing for erythropoietin (EPO) at the 2000 Games in Sydney. EPO has the same effect as blood doping, but it is less dangerous and messy because it involves an injection rather than blood transfusions.

Ljungqvist said the mistake in handling the sample did not suggest that the new test was ineffective or flawed.

The Phonak Cycling Team, however, said in a statement yesterday it was establishing an anonymous scientific board of experts to examine the reliability of the new test. The team said Hamilton, who was suspended Wednesday, would remain suspended but would stay with the team until the examination had been completed.

Phonak has already informed Hamilton that it would invalidate his contract if he should be found guilty of a doping offense. The UCI, which has not commented on the case, is expected to hand down a ban in the coming days.

Hamilton would then have the opportunity to appeal to the Court of Arbitration for Sport.

Hamilton, 33, who finished fourth in last year's Tour de France despite racing for three weeks with a broken collarbone, resides in Girona, Spain, with a large group of American cyclists. He was a member of Lance Armstrong's U.S. Postal Service Team for several years before leaving to become the star rider for Phonak.

An alpine skier in college at Colorado, Hamilton dropped out of this year's Tour de France after suffering a back injury.