The 16-year-old member of the girls' swim team had completed her fourth training lap when her coach's attention was momentarily diverted. When she looked again, the girl was floating face down in the water. Attempts to revive her failed.

A 9-year-old boy, swimming with friends at a summer camp, dived into a lake and failed to come up immediately. When he did surface, he was unresponsive and could not be resuscitated.

These types of deaths have long been a medical mystery. Listed as unexplained drownings, they have defied the efforts of coroners and doctors to understand how youngsters in seemingly perfect health could die.

After performing molecular autopsies on the children who died, Mayo Clinic researchers believe they may have found the answer: a rogue gene that can kill swimmers without leaving a trace.

"This is the first time that this gene has been caught in the act," said Michael Ackerman, who reported on his findings in the current issue of the journal Mayo Clinic Proceedings.

"It is the perfect electrical assassin because on autopsy there will be absolutely no clues," he said. "The heart will look pristine."

The finding may lead to a better understanding of the mechanism of cardiac arrest in general, as well as new treatments.

The gene, known as RyR2, regulates the influx of calcium into heart cells, which in turn regulate the electrical pulses that cause the heart to beat in a rhythmic pattern, said Ackerman, director of Mayo's Sudden Death Genomics Laboratory.

A mutation in the gene can upset the heart's rhythmic beating, a condition clinically called arrhythmia. And arrhythmia appears to be triggered in people with the defective gene by exposure to cold water, holding the breath, strenuous activity or emotional turmoil.

A heart that flutters instead of pumping blood deprives the brain of oxygen, causing a person to faint if the arrhythmia lasts a short time but leading to sudden death if prolonged, Ackerman said.

"When a person faints on dry land, he will likely survive," he said. "But when it happens in water, even if the heart restores normal rhythm, now you're drowning."

"This is an important development," said Arthur Moss, a cardiologist at the University of Rochester Medical Center, who is studying a second gene linked to sudden death, mostly in adolescents engaged in such sports as football and soccer.

The 16-year-old who drowned, Kristin Jacobs, had two earlier episodes, said her mother, Patrice Nelson of Berryville, Va.

"The first time, they said she had a seizure," said Nelson, an emergency room nurse. "The second time, they said she just fainted. The third time is when she went into cardiac arrest and died.

"Seizure and fainting are the most common misdiagnoses. She was 16 years old, and there was a tendency to minimize what was going on. But any teenager who faints during exercise or emotional upsets needs to have a certain kind of work-up for this mutation as a standard protocol, just like you would for chest pain."

Nelson has set up Kristin's Memorial Foundation to raise awareness in the medical profession and general public to a group of genetic mutations that can cause sudden death.

Ackerman estimates that the RyR2 mutation affects 1 in 5,000 to 1 in 10,000 people.

About 4,000 Americans drown each year, in addition to 20,000 near-drownings. Most of these cases are explainable -- accidents, alcohol use, strong currents -- but some are not.

"Of the drownings that are truly unexplained and don't make sense, we have every reason to believe that a good number of them could very well be this genetic disorder of the heart," Ackerman said.