Six children who died in a South African township earlier this month had ingested a chemical compound used in pesticides, the country’s health minister said on Monday, in a case that has prompted calls for tighter food safety controls.

Local media have reported that the children died after eating snacks bought from an informal cornershop in the Soweto township near Johannesburg.

“The cause of death is unequivocally organophosphate … a group of substances, which are usually used in agriculture or as pesticides,” Health Minister Aaron Motsoaledi told reporters as he announced the findings of a toxicology analysis.

Health officials are still waiting for the results of tests on swabs from informal township cornershops – known as spaza shops in South Africa – in Soweto as they try to establish where the organophosphate came from.

“Our theory, until we are proven otherwise, is that they (the shops) are using it as pesticides to kill rats … It is also being sold to communities for killing rats,” Motsoaledi said, adding that pesticide products containing organophosphate were not routinely meant to be used in people’s homes.