PATNA, India — The free school lunch that killed 23 Indian children last week was contaminated with a concentrated pesticide that is not widely available, the district magistrate overseeing the police investigation said Sunday.

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

The deaths set off protests in Bihar. The meal was part of India’s school lunch program, which 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.