A man mourns the death of his daughter in the eastern Indian state of Bihar on July 18. She died after consuming contaminated meals given to children at a school on Tuesday.

PATNA, India - The free school lunch that killed 23 Indian children last week was contaminated with concentrated pesticide which is not widely available, the district magistrate overseeing the police investigation told Reuters on Sunday.

The children fell ill within minutes of eating a meal of rice and potato curry in their one-room school in Bihar state on Tuesday, vomiting and convulsing with stomach cramps.

The deaths sparked protests in Bihar. 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. It had already drawn widespread complaints over food safety.

An initial forensic investigation found that the meal had been prepared with cooking oil that contained monocrotophos, an organophosphorus compound that is used as an agricultural pesticide, Ravindra Kumar, a senior police official, told reporters on Saturday.

The pesticide found in the oil was of a concentration more than five times that used in a commercial version, according to a forensic report.

"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 liter of monocrotophos is mixed with five liters of water."

Sinha said the concentrated form was not widely available and the pesticide was normally sold commercially in the diluted state.

Police said on Friday they suspected the cooking oil used in the meal 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 Organization describes monocrotophos as highly hazardous and that handling and application of it should be entrusted only to competently supervised and well-trained applicators.

The Food and Agriculture Organization of the United Nations says all waste and contaminated material associated with the chemical should be considered hazardous waste and destroyed in a special high temperature chemical incinerator facility. 

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