Indian police suspect that the country's worst outbreak of mass food poisoning in years was caused by cooking oil, the magistrate overseeing the investigation said.

"Circumstantial evidence suggests that cooking oil was kept in a container which was previously used to store pesticides or insecticides," Abhijit Sinha, district magistrate of Saran district, told the Reuters news agency on Friday.

Health officials in the eastern state of Bihar were due to release on Friday autopsy results for many of the 23 schoolchildren who died on Tuesday after vomiting and convulsing with agonising stomach cramps.

"At the moment we cannot say whether it was deliberate or it was pure negligence," Sinha said.

Officials said on Thursday it appeared from post mortem results that the food was contaminated with a pesticide.

The children fell ill within minutes after eating a lunch provided by their school in the village of Gandaman in Bihar.

Meanwhile, police and local officials raided the headmistress's home in Gandaman on Friday while grieving parents attacked the house to protest the deaths of pupils.

It is not yet clear where the headmistress bought the food for the free meals, cooked at a makeshift kitchen outside the one-room ramshackle school.

Police said she left the village on Tuesday with her husband, a local businessman who owned a shopping complex of about 40 stores.

India runs the world's largest school feeding programme, involving 120 million children, and Bihar is one of India's most populated and poorest states.

Educators see the scheme as a way to increase school attendance, in a country where almost half of all young children are undernourished. But children throughout the country often suffer from food poisoning.

In 1998, adulterated rapeseed oil killed as many as 60 people in the capital New Delhi. Investigations later revealed that the oil had been mixed with white oil, a petroleum product.