NEW DELHI — An intensive investigation of a mysterious annual epidemic in northern India in which thousands of young children suffer convulsions, lapse into comas and die has concluded that a toxin found in litchi fruit may be the cause.

“We believe it’s likely to be some sort of toxin” that causes a sharp drop in blood sugar levels that then leads to seizures, said Dr. Padmini Srikantiah, one of the authors of a description of the investigation in this week’s Morbidity and Mortality Weekly Report, a publication of the Centers for Disease Control and Prevention in the United States.

Investigators are testing a variety of possible poisons as catalysts for the malady, including pesticides and heavy metals. But the region is India’s litchi center, and the epidemic occurs every year just as the fruit ripen. So a toxin found in litchi seeds has become a focus of further testing, Dr. Srikantiah, a senior epidemiologist with the American agency, said on Thursday.

The affected children begin arriving every year in mid-May, brought to overburdened hospitals in one of India’s most impoverished areas by panic-stricken mothers who generally report that their young children, healthy just hours before, awoke with a scream in the middle of the night, suffered convulsions and then became unconscious.