London - An imbalance of a key brain chemical could cause crib death, researchers said on Thursday in what they called a chance discovery.

They created mice whose sudden deaths resembled crib death in humans, and found that the key may be an out of balance self-regulating system controlling the nerve-signalling chemical serotonin.

Writing in the journal Science, they said they hoped their experiment can help doctors pinpoint human babies at high risk of Sudden Infant Death Syndrome (SIDS), also known as cot death.

"At first sight the mice were normal," said Cornelius Gross of the European Molecular Biology Laboratory in Italy, who led the study.

"But then they suffered sporadic and unpredictable drops in heart rate and body temperature. More than half of the mice eventually died of these crises during a restricted period of early life. It was at that point that we thought it might have something to do with SIDS."

SIDS is a leading cause of death in babies under a year old in the developed world, yet its root cause remains a mystery. Healthy-looking infants can often die in less than an hour.

Smoking around infants and during pregnancy seems to be linked with SIDS, and campaigns to put babies to sleep on their backs instead of prone dramatically reduced crib death rates in several countries.

Cautioning parents about overheated rooms and minimizing bedding materials that could cause suffocation also may have reduced rates, but SIDS still kills one in every 2,000 babies globally.

"Ultimately, we hope it will give new ideas to doctors about how to diagnose babies at risk for SIDS," said Enrica Audero, a researcher who worked on the study.

In May, British scientists said they had pinpointed two common bacteria that may contribute to crib deaths, even when infants show no sign of tissue damage.

Gross said his team was studying the brain chemical's relationship to anxiety and aggression before they realized their results might have something to do with crib death.

Genetically engineered mice whose self-regulating serotonin system was turned off did not die, showing it was worse to have a malfunctioning system than no system at all, he added.

"We have actually shown you can introduce changes in the serotonin system and they can cause death," Gross said.

Serotonin neurons in the brainstem -- the part of the brain that controls breathing and other unconscious functions -- do not work properly in the gene-engineered mice.

Gross's team found that these mice died suddenly after they were unable to activate key brain functions, including the regulation of body temperature and heart rate.

While the researchers said it is unlikely the molecular mechanism is the same in humans, they believe their findings may one day help identify babies at greatest risk.