(CNN) -- At least 22 schoolchildren died in northeastern India after eating free school lunches that contained a poison, a state official said.
More than 25 others were hospitalized in Bihar state and three are critical, said Bihar Education Minister P.K. Shahi, after ingesting an insecticide that was in the food.
The poison was organophosphorous, a chemical that the U.S. Centers for Disease Control and Prevention says is commonly used in agriculture.
It is a nerve agent related to sarin gas, which is used in chemical warfare, the U.S. Health Department says.
Exposure to a high dose can cause an irregular heartbeat, difficulty breathing, paralysis and seizures.
A program providing one free hot meal a day to school children has proved incredibly popular as part of India's wider effort to tackle malnutrition. Children aged 6 months to 14 years get take-home rations or are provided with hot cooked food.
The wider $22 billion-a-year welfare scheme aims to sell subsidized wheat and rice to 67% of its 1.2 billion people.
According to the Indian government's figures, nearly half of India's children suffer from malnutrition of some sort.
Shahi said: "Twenty million children are being served hot meals in about 73,000 elementary schools. We have been endeavoring to improve the quality ... However, the challenge is still there because the magnitude of this program is so huge that there are a number of challenges."
He added: "It is really very unfortunate. Even though I would unhesitatingly admit that there are some quality issues before us, but this is the first incident which has happened in the state. In the past we have received complaints regarding quality, but the incident of this nature has happened for the first time. It has really shocked us -- shocked the entire state."
Speaking on CNN's sister network CNN-IBN, district magistrate Abhijit Sinha said an inquiry into the deaths had been launched.
CNN-IBN reported that the children were between the ages of 5 and 12 and from Dharamsati, a village in Saran district, Bihar state. It said their deaths Tuesday triggered violent protests Wednesday in Chhapra, the headquarters of Saran district, and a call from politicians for a general strike.
Madhusudan Paswan, Saran's district education officer, told CNN that 31 of the affected children were sent from the local Sadar Hospital to Patna Medical College Hospital in the state capital Tuesday night.
CNN-IBN quoted Shahi as saying the deaths were a clear result of poisoning, and an investigation would determine whether the contamination was accidental or deliberate.
Since a landmark Supreme Court decision in 2001, all government schools in India have been required to provide free meals to students younger than 13.
CNN's Sumnima Udas contributed to this report.