An initial forensic report has confirmed that the free school lunch that killed 23 children this week in India's eastern state of Bihar was contaminated with a pesticide, a senior police official has said.
The report found the meal was prepared with cooking oil that contained monocrotophos, an organophosphorus compound that is used as an agricultural pesticide, Ravindra Kumar, a senior police official, said on Saturday.
"It is highly poisonous, it's highly toxic, and, therefore, it has to be diluted when used as commercial pesticides," said district magistrate Abhijit Sinha.
"Typically it has to be diluted five times. So one litre of monocrotophos is mixed with five litres of water."
The children fell ill within minutes of eating a meal of rice and potato curry in their one-room school on Tuesday, vomiting and convulsing with stomach cramps.
The deaths sparked protests in Bihar and highlighted the negligence of authorities.
The lunch was part of India's Mid-Day Meal Scheme that covers 120 million children and aims to tackle malnutrition and encourage school attendance.
Complaints about the bad quality of food are widespread, and students in remote parts of India say the food itself is not the only problem.
Poor infrastructure has marred the scheme in some schools, which do not have a proper kitchens for cooking the meals and sanitary locations for children to eat.
Police said on Friday they suspected the oil was kept in a container previously used to store the pesticide.
They are still looking for the headmistress of the school, who fled after the deaths.
The World Health Organisation describes monocrotophos as highly hazardous.