The father of the child, an employee of the aeronautics company Safran, supposedly forgot to drop his child off at nursery in the morning before going to work.
The baby was found dead on Wednesday afternoon, when temperatures were around 22C, in the car park and an investigation has now been launched into his death.
“Initial investigations suggest that the young child died of suffocation and dehydration,” said public prosecutor Cecile Gensac in a statement.
His mother raised the alarm after she noticed her child was not at nursery late on Wednesday.
The fire brigade and an ambulance were immediately called to the scene and attempted to resuscitate the child around 5 pm.
“No one heard the slightest noise,” the public prosecutor said as he realised their attempts had failed.
The parents of the baby, who both had no previous criminal records and also have an older child, were sent to Pau hospital after suffering from “a state of shock”.
A psychological support unit has also been set up for the staff of the child nursery, which employs 26 people and looks after around 80 children.
The establishment was closed for two days said the mayor of the town, Jean-Christophe Rhaut.
An autopsy will determine the exact cause of death.