REUTERS/Adnan Abidi
The children died after eating lentils, potatoes and rice cooked at the school last Tuesday with oil containing an agricultural insecticide -- organophosphorus -- that was five times the strength sold in the market place, the report said.
"The report has found organophospharus in oil samples collected from the school where the mid-day meal was prepared and consumed by the children," Ravinder Kumar, a senior police officer, told reporters in Bihar state capital Patna.
"It was observed by the scientists of the Forensic Science Laboratory that the poisonous substance in the (food) oil samples was more than five times the commercial preparation available in the market," Kumar added.
Many of the 23 victims, aged four to 12, from Gandaman village in Bihar, were buried in a playing field adjacent to the primary school that served the free school lunch -- the only meal of the day for a number of the poor youngsters.
Some 24 children and a cook are still being treated in hospital, but medical officials said they were believed to be out of danger.
Kumar would not be drawn on whether the insecticide had found its way into the oil deliberately.
"It is a matter of investigation how and where this poisonous substance got mixed into the oil," he told reporters.