Preliminary investigations over the case which has resulted in angry protests in India indicate the cooking medium was mixed with pesticide, according to the Times of India.
Police say that the poisoning was most probably caused by storing cooking oil in a used pesticide container.
“Earlier it was thought oil was put in a pesticide container. But it seems that the lady cook used pesticide,” the source said.
The subzi looked black and smelt odd according to the source.
“The school principal insisted the cooking medium cannot be bad since it had come from her husband's shop,” he said.
“When students refused to eat, the principal scolded and forced them to eat.”
"I can accept accidents can happen anywhere. But the scale of this incident and the carelessness, the sheer callousness of the response, the delay in treating the children, is shocking," activist Dipa Sinha of the Right to Food Campaign told AFP.
The children died after eating lentils, potatoes and rice cooked with oil containing agricultural insecticide that was five times the strength sold in the market place, according to tests.
Forensic tests have shown that the oil contained a "highly toxic" form of insecticide which killed the children so quickly that some died in their parents' arms while being taken to hospital.
The principal reportedly fled the school shortly after realizing the extent of the tragedy.
There have been violent protests in the state of Bihar.
Several parents who lost children in the tragedy said they had trusted the school to serve their kids their only solid meal of the day.
"We have no food at home and it was only to ensure that my children got at least some food that I sent them to the school," a mother who lost three children in the tragedy, Sanjudevi Mahatoshe, told AFP.
Tens of thousands of children in India have been reportedly been refusing to eat their meals in the aftermath of the tragedy.
On top of this, there have been a number of reports of children taken to hospital, and one report of a scorpion found in the rice in a midday meal.
SHI/SHI