Two Norwegian doctors have claimed a major breakthrough in the struggle to solve the mystery of "crib death," which kills one or two of every thousand infants before their first birthday.

Torleiv Ole Rognum and Ola Didrik Saugstad say they proved the baby dies from a lack of oxygen.Their findings, published this month in the respected U.S. medical journal Pediatrics, are important because they discount a host of other theories about the sudden death of infants from no apparent cause.

"This is a major step," Rognum told Reuters in a telephone interview. "We have found the primary cause of death, although no one yet knows what limits the oxygen supply."

Rognum, who has worked on the problem for the last four years with Saugstad in Oslo, said crib death was the biggest single cause of infant mortality in the Western world.

"Somewhere between one and two of every thousand infants die of this before they are a year old and the numbers are going up," he said. "The problem is that an autopsy tells us nothing. Medical science has been baffled by it."

The two doctors found unusually large amounts of a chemical called hypoxantin in the eye fluid of infants who have died in this way. The body's production of hypoxantin rises sharply if the supply of oxygen is reduced significantly.

Their research, completed a year ago, showed concentrations of the chemical were six times higher in infants who had suffered crib death than in those killed by other known causes.

"This supports one of the many theories about crib death -- that it has something to do with respiratory problems -- and discounts a number of others," Rognum said.

Some medical researchers have suggested the infant dies because its tiny lungs do not work properly, while others have suggested that heart problems are the cause.