Tests at the children's home showed the presence of phosphine, a highly toxic chemical.
German prosecutors say that the misuse of pesticides may have led to the deaths of two young children near Frankfurt.
The bodies of a five-year-old girl and her four-year-old brother were found at their home in the town of Karlstein early on Monday. Emergency services were unable to revive the children after arriving at the scene.
Subsequent tests on 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 now say that this may have resulted in a chemical reaction that 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.