With the lawsuit dragging on, Linnie was losing hope. Rosen had found her a place in an off-reservation rehabilitation center in Colorado, but she yearned to return home. By this point, she wore diapers. "Mom can't handle you no more," Laura told her.

Then Linnie broke a Navajo taboo. She told Laura: "I just wanna die."

Laura remembers replying: "Linnie, don't say that. I don't want you to think that way."

In June 2000, the telephone rang in the middle of the night, and an older Neztsosie sister, Nora, answered. She told Laura that Linnie, just shy of 30, was gone. They held each other and cried.

El Paso had fought all the way to the U.S. Supreme Court in a successful effort to have the case moved from tribal to federal courts, where the nuclear industry enjoyed partial protection from liability. The federal court appointed a mediator.

Within a few months of Linnie's death, El Paso agreed to pay a total of $500,000 to the four families without admitting liability, tribal court documents show.

Justice presented his findings at a conference of the Public Health Service Commissioned Officers Assn. in 2001. Yet the view that Navajo neuropathy was purely inherited continued to have its adherents.

It did seem logical. There were multiple cases within families. The syndrome had appeared suddenly, the way a "founder effect" disease might.

A "founder effect" begins with one person who develops a genetic mutation in a nondominant gene. That person passes it on to his or her children, who pass it on to theirs. Because the gene is recessive, the trait does not surface unless two descendants have children together, usually generations later.

A team from Tufts University and another from Columbia University examined three genes that might cause the disease. But they reached a dead end each time.

Then the Columbia group found that liver tissue from three Navajo neuropathy patients showed reduced levels of mitochondrial DNA, a condition that can lead to progressive organ damage.

This year, in the September issue of the American Journal of Human Genetics, the Columbia group announced a breakthrough. An Italian scientist had found a previously unknown mutation in a recessive gene that caused mitochondrial disease of the brain and liver.

Testing DNA samples from six Navajo neuropathy patients, Columbia neurology professor Salvatore DiMauro and his colleagues found the same mutation.

Still, some aspects of Navajo neuropathy do not fit the genetic theory — or suggest that heredity is only one factor.

For one thing, Italians with the genetic mutation suffered liver disease, but not the curled hands or loss of sensation seen among Navajos.

After Navajo neuropathy appeared in 1959, reported new cases increased through the 1960s, '70s and '80s, then tapered off in the 1990s and have all but disappeared — an arc that mirrors Navajos' exposure to contaminated water from pit mines.

The increase in cases occurred while the mines were being abandoned and were filling with water. The drop-off roughly coincided with the filling-in of the pits by the tribal government.

If the illness was exclusively hereditary, "there should be more, not fewer, cases as the years go on," and then the numbers should level off, said Richard I. Kelley, a pediatric neurologist at Johns Hopkins University who has identified "founder effect" diseases among the Amish.

DiMauro said it was possible that, as with many diseases, a combination of genetic and environmental factors was responsible. "There are still things to be explained," he said.

Kelley said his review of the scientific literature and medical reports from the Neztsosies' lawsuit "has left me convinced that this is an environmental disease."