There have been violent protests in the Indian state of Bihar after 22 children died and dozens more fell sick after eating a tainted free school meal.
Four police vehicles were set on fire in Dharmasati Gandaman in Saran district during the protests by parents and hundreds of villagers.
A doctor at the local hospital said a chemical used in pesticides was likely to be the cause of the contamination.
The Mid-Day Meal Scheme provides free food, but often suffers poor hygiene.
It was introduced to combat hunger and boost school attendance, and reaches 120 million children in 1.2 million schools across the country, according to the government.
The school's cook told a government official she thought a new type of cooking oil was the cause of the poisoning.
Twenty-eight sick primary school children were taken to hospitals in the nearby town of Chhapra and the state capital, Patna, after consuming the meal of rice, soybeans and potatoes on Tuesday.
BBC correspondent Amarnath Tewary said local people armed with poles and sticks had blocked streets and locked railway gates, halting the movement of trains.
As well as the protests in Dharmasati Gandaman - where the school is located - our correspondent said a crowd set fire to a bus and damaged private property in Chhapra.
Political parties have called for a protest strike.
But, our correspondent said, political leaders have been chased from the streets by enraged protesters.
A total of 47 students of the primary school fell sick after eating the free lunch.
KM Dubey, who is doctor at the Chhapra district hospital, said: "When the children were clinically examined, we saw that there was severe congestion in the chest, and their pupils were dilated. These are symptoms of organo-phosphorus poisoning.
"Organo-phosphorus is a compound also used as a pesticide for crops. It is very dangerous. Even a small quantity of it would prove fatal for small children. Looking at the critical condition in which they were brought to the hospital, it seems like there were large quantities of poison in the food that they consumed."
State education minister, PK Shahi, told a news conference the school's cook was stable in hospital, having eaten a "small quantity" of rice.
The cook told him that the school had bought a new type of cooking oil and she thought the oil looked "discoloured and dodgy".
She drew it to the attention of the teacher - who was in charge of the Mid-Day Meal Scheme at the school and had been transferred there recently - who said the "oil was home-made and safe to use".
The minister said the oil had been bought from the grocery store run by this teacher's husband.
Mr Shahi said the grocery store owner and other men in his family had fled.
An inquiry has begun to find the cause of the poisoning and 200,000 rupees ($3,370) in compensation offered to the families of each of the dead.
There are fears the number of dead could rise as some of the children, all below the age of 12, are critically ill.
The father of one sick child, Raja Yadav, said his son had been vomiting after returning from school and had to be rushed to hospital.
Patna-based journalist Amarnath Tewary says villagers told local reporters that similar cases of food poisoning from Mid-Day Meals had happened in the area previously.
Mr Shahi, told the BBC "that food is not being checked before it is being served".
He added that "the scale at which the operation is being carried out, serving food to 20 million children every day and that too in remotest village schools, checking food before it is served - that itself is a challenge".
Bihar Chief Minister Nitish Kumar called an emergency meeting and ordered a team of forensic experts to the school.
Bihar is one of India's poorest and most populous states.
The Mid-Day Meal was first introduced for poor and disadvantaged children in the southern city of Chennai in 1925.