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 say they 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 Friday’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. Bacterial infections have long been suspected by some doctors to play a role in SIDS.

“We don’t know whether it’s a cause or if it’s identifying another potential risk factor,” said Dr. Nigel Klein, a professor at the Great Ormond Street Hospital for Children, where the study was conducted, and one of the paper’s authors.

He said that 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 in an otherwise healthy infant who dies suddenly, usually in their sleep. In the United States, SIDS kills more than 2,000 infants every year.

The researchers used autopsy samples from 470 infants who died suddenly and unexpectedly between 1996 and 2005. They found dangerous bacteria in 181 babies, or 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.

Most of the bacteria were detected in the babies’ lungs and spleens.