Pesticide was the most likely cause of death in 27 school children in the Philippines, the country's health ministry has said.
Many of the dead were laid to rest in a mass burial last week
The children died last week after eating a mid-morning snack of cassava roots at a school in the town of Mabini, southern Bohol province.
Officials earlier said the children could have been killed by naturally occurring cyanide in the roots.
But tests indicated pesticide may have contaminated the cassava.
"It is very much possible that the food was prepared in an environment that was highly toxic and contaminated with chemical poisons and bacteria," said Health Secretary Manuel Dayrit in a written statement.
"Dehydration secondary to pesticide poisoning was the main cause of death," he added.
One of the vendors who sold the snacks was among dozens of casualties admitted to hospital after eating the roots.
The other is in police custody.
Cassava root is traditionally boiled as a substitute for rice in some poorer areas of the Philippines, but in this case appears to have been fried and processed into snacks.
Victims suffered severe stomach pains, vomiting and diarrhoea.