Researchers believe they have discovered the cause of mysterious deaths that claimed the lives of several healthy children in two extended Amish families 15 years ago.

In 2004, two siblings died suddenly while they were playing, and over the years the same tragedy struck two of the family’s other children, leaving the community with unanswered questions.

When autopsies for the first two children did not identify an underlying cause of death, the local medical examiner’s office reached out to the Windland Smith Rice Sudden Death Genomics Laboratory at the Mayo Clinic. But the lab, which examines unexpected deaths, could not determine why the children died.

Over the years, though, the Mayo Clinic developed better tools, and researchers now say they believe they've made a major breakthrough in solving the mystery behind the deaths that also claimed the lives of children from a second family.

In a study published by the lab in Jama Cardiology this month, researchers examined DNA of the four Amish siblings and found they all had a duplication of genetic material called RYR2 that put them at risk. Mutations of the gene can cause a cardiac arrhythmic disorder that can lead to exercise-fainting spells, seizures or sudden cardiac death.

Researcher said the children had all inherited the same genetic mutation from both of their parents. Of the 23 young people who had inherited the mutation, 18 had died suddenly. Both families had an extensive history of having children who either died during physical activity or who survived cardiac events.

“We finally figured out that it was an autosomal recessive condition where both bad duplications came from both parents, and those children were unfortunate to get the double dose, “ Michael Ackerman, director of the Windland Smith Rice Sudden Death Genomics Laboratory, told CNN.

Researchers said the Amish community could be more vulnerable to the conditions because they come from a small number of ancestors who tend to intermarry.

Knowing about the genetic marker can help medical professionals take preventive steps to avoid similar deaths by testing an individual for the presence of the mutation, researchers said.