Virgile Bourdon, 35, was discovered unconscious this Monday morning in his individual cell in Mont-de-Marsan.
Resuscitation attempts by the supervisors, then by the emergency services, could not save him, said prosecutor Olivier Janson.
The detainee, awaiting trial, confessed in early 2019 to the murder of his companion Laura Fay, a 30-year-old Ardéchoise who disappeared in 2018 in the Landes.
An autopsy is due this week as part of a procedure to investigate the cause of death.
But the detainee "did not present any identified pathology", had been seen "the same morning, shortly after 7 am, in apparently normal state", underlined the prosecution.
In addition, the first elements of the investigation do not allow at this stage to favor the hypothesis of a suicide.
It "was not found writing in this direction" and the detainee "does not appear to have made suicidal remarks", specifies the prosecutor.
He was due to be tried in January
Virgile Bourdon was indicted in January 2019 for murder by spouse of Laura Fay, with whom he had been in a relationship for eight months.
The young woman, originally from Villeneuve-de-Berg (Ardèche), had disappeared in August 2018 in the north of the Landes.
His companion, a homeless man who carried out seasonal jobs, had initially been heard as a simple witness, then left free.
The hypothesis was then that after an argument between the couple, who was to go to a music festival in the Landes, she left alone with a friend living in the region.
A judicial investigation had been opened for kidnapping and sequestration, and the investigation had mobilized a dozen investigators, the gendarmeries of Pau and Grenoble, for months, many beatings and searches carried out in vain.
Virgile Bourdon was finally arrested in January 2019 at his mother's home in the Drôme. He had confessed to the murder in police custody, citing an argument that had gone awry. Then he had led the investigators to the place where he had hidden the body, in a sleeping bag, hidden in the vegetation of a forest road, a few kilometers from the place of the murder, Uchacq-et-Parentis. Virgile Bourdon was to be tried by the Landes Assize Court next January.