A newly discovered defect in the hearts of some babies may be responsible for half of all cases of sudden infant death syndrome, a major killer of infants, researchers say.

The baffling syndrome, also known as crib death, or SIDS, strikes about one in every 500 babies in the United States. It is the most common cause of death among infants 2 weeks to 1 year old.

The new evidence suggests that these babies' hearts frequently have a defect in their electrical stimulation. The malfunction could make their hearts stall when they begin to speed up for any reason.

The discovery does not provide immediate new ways to prevent crib deaths. But the researchers say it may someday help identify babies who are at high risk so they could be treated with drugs.

"It is not something that will save lives next year, but it's a clue that there is something defective in the way the nervous system controls the heart," said Dr. Daniel C. Shannon, a co-author of the study at Massachusetts General Hospital.

He speculated the defect could result from immaturity or from damage triggered by an infection or toxin encountered as a fetus.

Sudden infant death syndrome has long been a medical enigma. Seemingly healthy babies die without warning, often in their sleep. Some clearly stop breathing because of a respiratory defect, but Shannon said they probably account for less than 10 percent of all cases.

The latest research, published in Thursday's New England Journal of Medicine, substantiates a theory proposed by Dr. Peter Schwartz of the University of Milan in Italy more than 10 years ago.