Sudden infant death syndrome (SIDS) and sudden unexplained child death (SUDC) are responsible for the deaths of more than 3,000 babies and children in the U.S. each year, the majority of whom are less than a year old. Although there are ways to reduce the risk of SIDS, no one knows precisely why it occurs. But a breakthrough new study sheds light on the possible “smoking gun” behind SIDS and SUDC.

For the study, published in the journal Neurology, researchers analyzed rare home video recordings — either from security systems or commercial crib cameras — of seven toddlers between ages 1 and 3 on the night or afternoon they died from SUDC, and for whom autopsy did not reveal a definitive cause of death. They also analyzed the children’s medical histories. The videos came from a registry of more than 300 SUDC cases from the NYU Grossman School of Medicine.

The researchers found a commonality between all but one of the cases: seizures, as evidenced by muscle convulsions. For all of the children, the seizure lasted for less than 60 seconds and occurred within a half hour of death. Only one had a history of febrile seizures, or a seizure in a child that’s caused by fever.

Without the videos, there would have been no reason to suspect that seizures were a potential cause of death in these cases.

“Our study, although small, offers the first direct evidence that seizures may be responsible for some sudden deaths in children, which are usually unwitnessed during sleep,” lead investigator Laura Gould, a research assistant professor at NYU Langone, said in a press release. Gould lost her 15-month-old daughter to SUDC in 1997, a tragic event that prompted her to establish the SUDC registry.

Previously, researchers had noticed that many children who died from SUDC had a history of febrile seizures, which are accompanied by a fever. In fact, children who pass away suddenly and unexpectedly are more than 10 times more likely to have had febrile seizures than children who died from a cause other than SUDC. At NYU Langone Health, medical records show that a third of children who have died from SUDC had a history of febrile seizure.

“These study findings show that seizures are much more common than patients’ medical histories suggest, and that further research is needed to determine if seizures are frequent occurrences in sleep-related deaths in toddlers, and potentially in infants, older children, and adults,” study senior investigator and neurologist Orrin Devinsky, M.D., said in the press release.

“Convulsive seizures may be the ‘smoking gun’ that medical science has been looking for to understand why these children die,” Devinsky added.

Further research is needed to confirm these findings and determine how seizures with or without fever may lead to sudden death in young children. Ideally, more video recordings and more comprehensive health records of children who die from SUDC will become available for future studies. However, the new study, although small, is noteworthy for being the first direct evidence linking seizures to SUDC.

Previous research in people with epilepsy suggests that difficulty breathing immediately following a seizure could be the link between them and death, Devinsky said.

About one in every 25 children will have a febrile seizure, and about 40% of children that have one will have another, according to the National Institute of Neurological Disorders and Stroke. Risk peaks at age 2. These seizures are usually triggered by fevers about 101 degrees Fahrenheit that occurs with an illness such as the flu, a cold, or an ear infection.

Gould doesn’t want the study’s findings to worry parents whose babies develop a fever or have a history of febrile seizures. It doesn’t change any advice in regard to these conditions, and SUDC is still rare.

Future research into the causes of SUDC could lead to additional strategies to decrease the risk of it occurring. For now, experts recommend placing your baby to sleep on their back, keeping their mattress bare without toys or blankets, making sure your baby isn’t overheated while sleeping, breastfeeding for at least six months, and offering a pacifier during bedtime and naptime.