Investigators examining the fiery crash site in southwest France where a truck and a bus collided say it will take three more days just to recover the charred bodies of the 43 victims.

Col. Patrick Touron of the Criminal Research Institute of the Gendarmerie joined other investigators Saturday on a winding road near the village of Puisseguin, near Bordeaux.

Touron said the cause of Friday's crash remains unknown, because the truck's black box is too damaged for data to be immediately recovered.

French President Francois Hollande will lead a ceremony in Paris on Tuesday to honor the victims.

The crash killed 41 bus passengers, most of them local retirees on a day trip, the truck driver and his 3-year-old son. Four other people were badly injured.