Patna, India (CNN) -- A week after an Indian school served toxic food to students, leaving 23 dead, its headmistress remains missing along with her husband, police said Tuesday.
A nine-member team of officers has been formed to investigate and track down the principal, Meena Kumari, police superintendent Sujeet Kumar said.
Police presence is heavy in the village in Bihar state, especially around the principal's home.
Authorities have recorded statements from 40 witnesses, including child survivors of the July 16 food poisoning, Kumar said.
Residents went on a rampage a day after the toxic meals were served in the local government school, torching at least four police cars.
In acts of protest, parents of at least three children have buried their kids near the school -- one right in front of the building, according to officials.
Police will ensure the headmistress' safety when she resurfaces or is taken into custody for questioning, authorities said.
Pesticides have been found in the food and oil used in the school lunch that sickened 25 others on July 16 in northern India's Bihar state, police said.
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.
The cook, Manju Devi, was hospitalized after eating the food she prepared, doctors said.
Devi told police that the headmistress did not heed her warning that the mustard oil used to prepare lunch looked and smelled bad. Instead, the headmistress insisted she continue preparing the meal, officials said.
An investigation found compromised hygiene and sanitation in the school, which was running from a single-room makeshift building.
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 the state's school meal program.
Authorities in Bihar -- one of India's poorest states -- announced that a committee would be formed to improve food preparation in rural schools.
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 from Putna and Tom Watkins wrote it from Atlanta