Dashing the hopes of many people with chronic fatigue syndrome, an eagerly awaited study coordinated by government health agencies has not confirmed a link between the illness and a virus called XMRV or others from the same class of mouse leukemia viruses.

Two research groups had earlier reported an association between chronic fatigue syndrome and the group of viruses, known as murine leukemia viruses, or M.L.V.’s, raising hopes that a treatment or cure could be found. But later studies did not substantiate the link, and many researchers suggested that that the initial findings were the result of contamination of laboratory samples or equipment.

The new multilab study, published online Thursday in the journal Science, was designed to answer some of the questions about these unusual viruses and determine whether they posed a risk to the blood supply.

Results from another government-sponsored study of M.L.V.’s, with a much larger sample size, are expected early next year. But Thursday’s report appeared to leave little room for continued optimism.

Of the nine labs that received blinded blood samples from 15 people previously reported to have been infected with M.L.V.’s and 15 healthy controls, only two reported finding evidence of the viruses in any of the samples. And the results from those two labs - which were the only two to find positive results in the original studies — contradicted not only each other this time, but some of their own earlier findings as well.

“These results indicate that current assays do not reproducibly detect XMRV/M.L.V. in blood samples and that blood donor screening is not warranted,” reported the new study, written by researchers participating the Blood XMRV Scientific Working Group.

But the scientists also said they could not “definitively exclude” the possibility that levels of viral markers in the blood might fluctuate over time and become undetectable at certain periods.

Also on Thursday, researchers from the original study linking XMRV to chronic fatigue syndrome, which was published in Science in October 2009, retracted a portion of their data — but not their conclusions — because of evidence of contamination in one lab involved in the study.

Vincent Racaniello, a microbiology professor at Columbia University who has covered the controversy on his popular virology blog, said the XMRV/M.L.V. hypothesis was now dead. “It’s clearly time to move on in the study for the origin of this disease,” he wrote in an e-mail message.

An estimated one million Americans suffer from chronic fatigue syndrome. Countless studies have documented immunological, neurological and other physiological abnormalities. Despite the name of the illness, patients have long reported that simple fatigue is not their cardinal symptom but rather what researchers call postexertional exhaustion — a profound depletion of energy after even minimal exercise or activity.

Recently, a panel of top researchers proposed a new definition of the illness that requires the presence of postexertional exhaustion, rather than the six months of unexplained fatigue required under the standard definition.

The group also recommended changing the name to myalgic encephalomyelitis, a virtually identical illness long recognized by the World Health Organization.

Dr. Nancy Klimas, an immunologist at the University of Miami, said that the two-year debate over M.L.V.’s had raised the profile of the disease and brought attention to the likely role of infectious agents in chronic fatigue syndrome.

“Internationally recognized experts have looked at the immune data and concluded that there very well may be a pathogen or pathogens involved in the persistence of this illness,” Dr. Klimas wrote in an e-mail message.

The new findings will also be presented Friday in Ottawa at the annual conference of the International Association for Chronic Fatigue Syndrome/ME, a leading scientific organization.

This article has been revised to reflect the following correction:

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By DAVID TULLER