Toulouse prosecutor Dominique Alzeari said 19 of the 82 residents of the retirement home remained under medical supervision, 16 of whom were in very poor condition although their lives were not thought to be in danger.
22 people at the privately operated Cheneraie residence in Lherm, a town south of Toulouse, began showing symptoms including vomiting after dinner on Sunday, police said, and five people are now confirmed to have died.
Alzeari said investigations into apparent cases of "involuntary homicide and unintentional injury" would be lengthy, and underscored the "serious" nature of the affair.
Flowers left at the Cheneraie retirement home in Lherm, near Toulouse. Photo: AFP
The meals involved have been kept for analysis, the regional health agency said, and residents were being questioned about what they ate.
The grand-daughter of one 95-year-old woman who died said she had been served a Perigord salad, a regional speciality that includes duck, ham and foie gras.
"I still have the menu in my handbag and I know they had Perigord salad last night. What could that mean? Could it be the foie gras? They're going to do an autopsy in any case," the woman told the Depeche du Midi newspaper.
The daughter of two other residents, who gave her name as Chantal, told journalists outside the home that "apparently the problem was with meals prepared specially for palliative care patients."
The deaths come as the government prepares a bill on retirement financing for an ageing population of baby boomers that is expected to add nine billion euros to the national health budget by 2030.