The deaths in 2012 were first linked to toxins in the lychee fruit.
Controversial insecticide endosulfan, which has been banned by more than 80 countries, was responsible for the deaths of 13 children in Bangladesh in 2012, a study found on Monday.
All of the fatalities, caused by brain inflammation, were linked to exposure to lychee fruit and occurred within 20 hours of the symptoms surfacing, according to the research published in the American Journal of Tropical Medicine and Hygiene.
Similar deaths due to acute encephalitis syndrome (AES) have been recorded in lychee orchards in neighbouring India – though according to an analysis published in The Lancet these were caused by a naturally-occurring toxin in the fruit's seeds and pulp.
But the authors of the new study said the Bangladesh deaths, which baffled investigators, were not connected to the fruit.
"Our investigation suggested the seeds might not be the cause as the seeds are not eaten in Bangladesh and instead found the deaths in 2012 were most likely due to an exposure to multiple, highly toxic agrochemicals," said study lead author M Saiful Islam, an associate scientist at the International Center for Diarrhoeal Disease Research.
"These deaths occurred at a time when lychee was being harvested and consumed across Bangladesh. If the seeds were the cause, then we would expect to see cases scattered across the country, not just in a certain small area."
The study, which also involved researchers from the US Centers for Disease Control and Prevention, was based on an investigation into 14 cases of AES in children one to 12 years old that occurred between May 31 and June 30, 2012 in northern Bangladesh. Only one child survived.
Endosulfan was phased out by the US at the end of 2016 and by the European Union in 2005.