An invisible killer stalked the waters of Lake Powell for years, causing nearly half of the "drownings" reported there. But they were not simple drownings. They were part of a mystery that saw excellent swimmers unexpectedly sink and die, sometimes after just moments in the water.

Their real killer was carbon monoxide (CO) from the exhaust of engines and houseboat generators, sometimes found in concentrations so high behind boats that lethal amounts could be inhaled literally in seconds, making victims fall unconscious and drown or die from CO poisoning itself.

Stubborn detective work identified the problem and made initially reluctant agencies act to correct boat design. But continuing work begun at Lake Powell suggests carbon monoxide still may be an unrecognized mass killer nationwide.

"There could be as many as 250 boat-related drownings per year (nationally) that are carbon monoxide poisoning first," says Jane McCammon, one of the scientists who identified the problem.

"But that is a big guess," she adds, saying other areas often do not test for carbon monoxide poisoning in blood of "drowning" victims as doctors at Lake Powell have learned to do. So she extrapolates from work at Powell and by tracking a growing list of accidental drownings nationwide that show signs of CO poisoning.

"I have no doubt that the number (of CO boating deaths) that are actually happening is much, much larger than the number that is being identified and reported," she told the Morning News.

Dr. Robert Baron, medical adviser for Glen Canyon National Recreation Area, said the saga began when medics in the early 1990s started receiving calls for people who had passed out. "By the time they arrived, they had fully regained consciousness but couldn't remember anything," he said.

Many of them had been at the rear of boats, and officials were unsure what was happening. A medic suggested it was CO poisoning. Baron questioned how that could be for boats that were stationary with no motors running. The medic said houseboats had generators for air conditioning and electricity, and they vented into water at the rear of boats.

Baron said the first confirmed case came in 1995 when a 12-year-old drowned at the rear of a houseboat, and an autopsy found high CO levels in his blood. Baron then began years of pushing the Coast Guard and National Park Service to help with more detailed investigations and to campaign for a redesign of boats.

Things changed in 2000 when two boys from Colorado — 11-year-old Dillon Dixey and his 8-year-old brother, Logan — went swimming under the rear swimming platform on their houseboat on Lake Powell as a generator pumped exhaust under it.