High levels of agricultural insecticide have been found in samples taken from tainted food blamed for the deaths of 23 schoolchildren in north-eastern India.
Forty-seven were taken ill after eating a free meal of rice and soya beans at a school in Bihar state on Tuesday.
Police said "very toxic" levels of the pesticide monocrotophos had been detected by scientific tests.
Vegetable oil used to prepare the food was revealed to be highly contaminated.
"It was the high quantity of monocrotophos insecticide found in the food which proved fatal for the schoolchildren", said Ravindra Kumar, a top police official in Bihar state capital Patna.
He was speaking after forensic science experts in Saran district issued the results of tests carried out on the cooking oil.
On Friday, police said they suspected the vegetable oil had been in a container previously used to store the pesticide.
The school's cook, Manju Devi, had earlier accused the school principal, Meena Kumari, of forcing her to use the oil, despite the cook complaining that it "smelled strongly" and looked "dodgy".
Meena Kumari, who is still at large, is wanted on suspicion of criminal negligence, justice authorities say.
Saran District Magistrate Abhijit Sinha told the BBC that if she failed to turn herself in to police, her property would be seized.
The children, aged between five and 12, have been buried in and around the school in the village of Dharmasati Gandaman.
Angry parents say they want the graves to serve as a reminder that the children died due to state negligence.
On Wednesday, hundreds of parents and villagers protested by setting four police vehicles on fire.
The children fell ill with vomiting and stomach cramps moments after eating the meal.
The free school meals, know as the Mid-Day Meal, were first introduced for children from poor backgrounds in the southern city of Chennai (Madras) in 1925.
One of the biggest such schemes in the world, 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.
However, there have been frequent complaints of poor food hygiene.