Two young children, aged two and four, were found dead inside their family's car in the southern French town of Carpentras on Monday.
According to initial findings, the siblings had accidentally locked themselves inside the vehicle, the local prosecutor's office said.
The children's mother found them in the car, which was parked in the driveway of the family home. Emergency services attempted to resuscitate the children but were unable to save them.
The prosecutor's office has opened a preliminary investigation into involuntary manslaughter. A post-mortem examination has been ordered to establish the exact cause and circumstances of the deaths.
The 33-year-old mother was treated by firefighters for shock. Investigators were initially unable to question her because of her condition, authorities said. The investigation is ongoing.
The tragedy occurred as France endures an intense heatwave. Temperatures have reached as high as 42 degrees Celsius in parts of the country.
In Carpentras, the mercury climbed to around 39 degrees on Monday, according to the national weather service, Météo-France.
Large parts of France remain under the highest-level red heat alert, with temperatures expected to rise further in the coming days.
