TUCSON, Ariz. —  A three-year study into a cluster of childhood leukemia cases in Sierra Vista detected no toxic exposures that could have caused the illnesses, federal health officials said.

"We don't know why this cluster occurred. We really wish we had the answer, but we don't," Beverly Kingsley, an epidemiologist with the federal Centers for Disease Control and Prevention, said Thursday.

At least a dozen Sierra Vista children have developed this cancer of the blood and bone-marrow system since 1997 and two have died of it.

That's nearly triple the childhood leukemia rate expected in a town of 40,000 during that time period.

"We find nothing to indicate that any action needs to be taken in Sierra Vista," Kingsley said.

The CDC report said most of the 128 substances measured in the bodies of these children and their families "were low, and often lower than levels usually detected in the U.S. population."

However, the CDC did find a variation in a gene that controls how the body converts an unsafe chemical to a safe one in all of the leukemia-stricken children and almost half of the healthy children in the study.

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Investigators said they didn't know what effect this might have on the risk of leukemia.

Stressing that this is a new finding, CDC officials said they will launch larger studies of the gene variance.

"I've been in Sierra Vista for a long time and I've been watching both kids and adults die of leukemia since 1974," said Sue Ivory, a close friend of two families who lost children to this cancer. "We just keep attending these funerals. There's something wrong here. Everyone knows someone who has leukemia or cancer. This has been going on for a long time."

Ivory and several others at a public meeting the CDC held Thursday night in the military town southeast of Tucson expressed concern that the study didn't include enough victims, didn't span a long enough period of time and didn't include environmental testing for toxins.

In 2004, the CDC decided to draw and analyze bio-samples of blood, urine and cheek swabs from the surviving leukemia victims and their families, test them for 128 contaminants _ including toxic chemicals, metals, pesticides and volatile organic compounds _ and compare them with samples from healthy Sierra Vista children. The samples also underwent genetic testing.

Environmental toxins long have been suspected as a cause of trigger for leukemia, although only one _ the solvent benzene _ has ever been proved to cause it, authorities said.

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Information from: Arizona Daily Star, http://www.azstarnet.com

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