LONDON — A baffling phenomenon known as sudden infant death syndrome is one of the leading causes of death for children under 1. Now British researchers may have found a contributing factor: bacteria.

They found potentially dangerous bacteria such as Staphylococcus aureus and E. coli in nearly half of all babies who died suddenly and without explanation over a decade at a London hospital. Their findings are in today's Lancet medical journal.

The researchers cautioned, however, that while the bacteria were found in the SIDS babies, that does not necessarily mean the bugs were responsible.

"We don't know whether it's a cause or if it's identifying another potential risk factor," said Dr. Nigel Klein, one of the paper's authors. He said the higher level of bacteria might be evidence of another condition that killed the baby, such as a room that was too hot or had poor ventilation. Or it may have been coincidental.

A SIDS diagnosis means that no other cause of death can be found. They found dangerous bacteria in 181 of 470 autopsy samples from infants, nearly half of the 365 whose deaths were unexplained. There were similar bacteria in about a quarter (14 of 53) of the babies who died of known causes, excluding those who died of bacterial infections.

SIDS typically strikes between the ages of 8 and 10 weeks — about the same time that the antibodies babies received from the mother before birth have nearly run out and before the babies typically have started producing enough of their own.

That could make them particularly vulnerable to infections, said James Morris, a pathologist at the Royal Infirmary in Lancaster, who co-authored a commentary in the journal.

Willinger suggested that bacterial infections might simply aggravate other SIDS risk factors, such as smoke exposure or babies sleeping on their stomachs.