Carolyn Scully was a perfectly healthy baby - until the day she died.

"I fed her that morning and to think, hours later she was gone. It was inconceivable. It was the most horrific day of our lives," said Carolyn's mother, Katie.

"It all came so suddenly. And it changes your world," said Steve Scully.

Carolyn died of sudden infant death syndrome, or SIDS. It's the leading cause of death in babies between two and four months old. Babies that seem otherwise normal go to sleep and never wake up.

"That level of unknown and mystery is so hard for parents who have lost a baby. And now, some of that mystery has been removed," said Marian Willinger of the National Institute of Child Health and Human Development.

A study in November's Journal of the American Medical Association provides more evidence that brain abnormalities may cause SIDS.

Researchers found those babies had fewer receptors to process the chemical serotonin, which helps coordinate breathing, blood pressure and temperature.

They discovered the babies had more immature nerve cells, making it harder to wake up or turn over.

"What this finding says is, it's not your fault. This baby had a problem in their brain," said Willinger.

That's reassuring news for parents.

"As a parent you just feel that if anything goes wrong that you're to blame," said Katie.

Now science is proving it wasn't their fault.

Researchers hope more studies like this will help doctors identify which babies are at risk so that treatments can be developed.