BOSTON -- Local researchers have some new answers into the leading killer among babies under 1. The cause of sudden infant death syndrome has remained a mystery for years.

But as NewsCenter 5's Heather Unruh reported Tuesday, doctors at Children's Hospital Boston say a chemical inside the babies' brain may put certain children at higher risk.

"Without warning, old grief strangles me like it was new," said Kathy Whelen, of Westwood, Mass.

Kathy Whelan fought back her tears as she read from a poem she wrote about her first born son, Mikey.

"I swallow back bitter bubbles and hide my grief," she said.

Twenty years after 10-week old Mikey unexpectedly died in his bassinet, Whelan still struggles with grief.

"I remember how he smiled. I loved to spend time reading to him and singing to him," she said. "He was just so apparently healthy and then the next day, he just dies."

Mikey died from SIDS. Despite decades of research, it's still the number No. 1 killer of infants under 1. Studies have shown placing babies on their backs, keeping stuffed animals and blankets out of the crib, and avoiding smoke around the baby can reduce their risk of SIDS. But researchers have remained stumped about why SIDS strikes some babies and not others.

On Tuesday, researchers at Children's Hospital Boston released a study with some answers.

"We found that babies who died of SIDS had abnormalities in serotonin in regions of the brain stem that control breathing and heart rate and blood pressure during sleep," said Dr. Hannah Kinney, a researcher at Children's Hospital Boston. "A normal baby could respond to that challenge, lift its head up, turn its head and arouse or wake up. But a baby who has a defect in those brain stems circuits that use serotonin can't do that when challenged and they go on to die."

Mary McClain is a registered nurse who counsels families who have lost their babies at the Massachusetts Center for SIDS at Boston Medical Center. She is optimistic about this latest research.

"The next steps will be how to devise a test that will determine serotonin levels in an infant, in a newborn, and of course ultimately will lead to some sort of treatment," McClain said.

For Whelan, the research gives her hope knowing one day other parents won't have to live through the kind of tragedy her family did.

"When a baby dies, it's not just the life of the baby, it's the hopes and the dreams of the baby that dies," she said.