BERLIN—Prosecutors in Germany said Thursday that misuse of pesticides may have led to the deaths of two young children whose bodies were found in their home near Frankfurt this week. First responders arriving at the house in the town of Karlstein early Monday were unable to revive the five-year-old girl and her four-year-old brother. Tests in the building showed the presence of phosphine, a highly toxic chemical. The children’s 49-year-old father told investigators that he had used pesticides in the house. Authorities said this may have resulted in a chemical reaction which produced phosphine, though the precise cause of the children’s death hasn’t yet been determined. Prosecutors said the father is being investigated on two counts of causing death by negligence.