It was still something of a mystery why so many babies had died in inclined sleepers, even after millions of the popular products were recalled for safety concerns earlier this year.

The Consumer Product Safety Commission didn't have a good explanation. That's one reason why the agency allowed inclined sleepers including Fisher-Price's Rock 'n Play to continue being sold despite a rising death toll. At one point, the CPSC blamed the problem on parents failing to properly strap in their babies. The agency knew of at least 32 deaths by the time Fisher-Price pulled the product from the market in April - sparking public outrage over both the delayed reaction by regulators and the loss of a product that many parents said actually got their babies to sleep.

The death toll eventually reached 59 babies -- and still no clear explanation for what went wrong, the CPSC said.

But a new study by an outside expert hired by the CPSC suggests that babies died in inclined sleepers for exactly the reasons that pediatricians and safety advocates had been warning about for years. Its findings highlight CPSC staff's failure to understand the risk and undercuts arguments from industry officials that the product category could be made safe with minor changes to voluntary safety codes.

The study led by Erin Mannen, assistant professor of orthopedic surgery at the University of Arkansas for Medical Sciences who specializes in infant biomechanics, found that the product's design is dangerous.

Newborns are especially susceptible to suffocation in an inclined sleeper because the products appear to make it easier for babies to roll into an unsafe face-down position and puts them in an exhausting fight to maintain a safe posture. The study examined how 10 infants moved in the devices and monitored their blood-oxygen levels.

None of the inclined sleep products that Mannen tested were found to be safe for infant sleep.

"I think this confirms what we've been saying all along," said Nancy Cowles, executive director of the safety advocacy group Kids in Danger.

Inclined sleepers have been controversial ever since Fisher-Price invented the category 10 years ago. The devices allowed babies to sleep at a 30-degree angle, violating the American Academy of Pediatrics' safe-sleep guidelines that said babies should sleep on flat surfaces. And Fisher-Price had invented its inclined sleeper based on faulty beliefs about infant sleep and without medical safety testing or input from a pediatrician, according to a Washington Post investigation.

Mannen was hired by the CPSC last year as the number of deaths associated with inclined sleepers grew, but staff struggled to pinpoint the problem.

Her study was included in a new CPSC proposal for mandatory safety rules for inclined sleepers. The new rules would effectively ban current models of inclined sleepers.

New sleepers would be limited to a 10-degree incline, the same limit for bassinets. Mannen's study found that the suffocation risk to infants disappeared below 10 degrees.

This article was written by Todd C. Frankel, a reporter for The Washington Post.