Brain abnormalities which affect breathing and temperature control could increase a baby's risk of cot death, a study has suggested.
Babies should be put to sleep on their back to reduce cot death risk
US researchers found infants who died of sudden infant death syndrome (SIDS) had flaws in their brainstem.
Writing in the Journal of the American Medical Association, the team said this meant the babies were unable to process the brain chemical serotonin.
UK experts said the study was a useful piece of a "jigsaw" of evidence.
The safest place for a baby to sleep is in a cot by the side of the parents' bed for the first six months
You can take your baby to bed for a feed or cuddle - but never leave it there
Put your baby to sleep on its back
Around 300 babies under the age of one die each year in the UK from SIDS.
The US team were building on previous research which had also suggested there were faults within the brains of babies who die from SIDS which make them more vulnerable to environmental factors such as infection or overheating.
In this study, the researchers from the Boston Children's Hospital and Harvard Medical School compared tissue from 31 babies who had died from SIDS with 10 who had died of other causes between 1997 and 2005.
As well as breathing, the lower brainstem helps control heart rate and blood pressure.
The researchers found that brainstems from babies affected by SIDS contained more nerve cells that make and use serotonin than the brainstems of the other infants.
But there were fewer serotonin receptors in SIDS babies' brains, meaning they could not process the brain chemical as they should.
Dr Hannah Kinney, who led the research, said: "These findings provide evidence that SIDS is not a mystery but a disorder that we can investigate with scientific methods, and some day, may be able to identify and treat."
A spokeswoman for the Foundation for the Study of Infant Deaths said: "The findings in the new paper are important, taken with previous reports.
"But this is unlikely to be the only inherited or non-modifiable risk factor."
She added: "Although the causes of sudden death in infancy remain unknown, it is generally agreed they are multi-factorial, and the present paper adds useful information.
"But much more research is needed in order to understand and, ultimately, prevent these tragedies, which claim about 300 babies' lives each year in the UK."