Patna, India (CNN) -- Pesticides have been found in the food and oil used in a free school lunch that killed 23 students and sickened 25 others on Tuesday in northern India's Bihar state, police said Saturday.
Forensic scientists found monocrotophos, an organophosphorus compound used as an insecticide, "in the samples of oil from the container, food remains on the platter and mixture of rice with vegetables in an aluminum utensil," Assistant Director General Ravinder Kumar told reporters in Patna.
Monocrotophos, which is used for agricultural purposes, is toxic to humans.
An administrative inquiry has pointed to negligence by the school headmistress in supervising food preparation for the children, Bihar state's midday meal director R. Lakshamanan told CNN on Friday.
The cook, Manju Devi, was hospitalized after eating the food she prepared, doctors said.
Devi told police that the headmistress, Meena Kumari, did not heed her warning that the mustard oil used to prepare Tuesday's lunch looked and smelled bad and instead insisted that she continue preparing the meal, Lakshmanan said, citing the inquiry report.
Police told CNN that investigators were trying to find Kumari to question her.
The investigation found compromised hygiene and sanitation in the school, which was running from a single-room makeshift building, he added.
Experts have said the deaths underscore the problem of food safety in the country and have prompted discussion on how to improve national school food programs amid news that authorities had warned months ago of safety problems with Bihar's school meal program.
On Friday, authorities in Bihar -- which is one of India's poorest states -- announced that a committee would be formed to improve food preparation in rural schools. Lakshamanan said village communities would help monitor standards for school meals.
The Bihar students, who were ages 5 to 12, started vomiting soon after their first bite of lunch; some fainted, authorities said.
According to the Indian government, nearly half of India's children are malnourished. Since a Supreme Court decision in 2001, government schools in India have been required to provide free meals to students younger than 13.
CNN's Harmeet Singh reported this story from Putna and Tom Watkins wrote it from Atlanta