Laboratory tests show that 27 schoolchildren who died after eating cassava roots last week were poisoned with pesticide, health experts said Monday.

Authorities earlier said they suspected the victims from Mabini (search) town on central Bohol island were sickened with cyanide poisoning that occurs if the starchy cassava roots are not properly cooked.

But Health Secretary Manuel Dayrit (search) told a news conference that tests showed pesticide was likely the cause.

"This means that it's very much possible that the food was prepared in an environment that was highly toxic and contaminated with chemical poisons and bacteria," Dayrit said.

Toxicologists Lyn Panganiban and Irma Macalinao of the University of the Philippines (search) and epidemiologist Troy Gepte said tests on cassava snack samples as well as an analysis of the response of patients to treatment indicated carbamate pesticide may have contaminated the cassava.

Carbamate pesticide is a commonly used in farms and households on Bohol and may have already been in the environment, they added.

More than 100 children who survived responded to atropine and rehydration, the experts said.

Gepte said health officials need to continue the investigation in coordination with the National Bureau of Investigation and other government agencies to conclusively determine how the poisoning occurred.

Vendors who sold sweetened cassava roots as recess snacks to the children — most ages 7 to 13 — last week denied that they improperly prepared the food.

One of the two vendors was among 103 patients who were admitted to four hospitals. The other was in police custody to protect her from outraged parents. Authorities have not ruled out criminal charges.