Nordahl Lelandais, 38, was brought from prison to the courthouse in Chambery, a town in the French Alps where the victim, another soldier, was last seen after leaving a nightclub.
He has admitted to killing Corporal Arthur Noyer in the early hours of April 12th, 2017, after picking him up as he hitchhiked.
Lelandais has also confessed to killing eight-year-old Maelys de Araujo in August 2017, in a case that horrified France and which is set to go to trial next year.
He has insisted both deaths were accidental but his confessions prompted the reopening of investigations into dozens of other killings to see if he was linked to these unsolved cases.
More than 100 journalists watched as he entered under heavy police escort, wearing a face mask and a dark cap pulled low over his eyes.
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Lelandais told police that a fight broke out with Noyer after they stopped at a parking spot, which he claims Noyer started, but Lelandais said he did not intend to kill him.
But prosecutors say he later put the victim’s unconscious body in the trunk of his car and drove it some 20 kilometres away before dumping it on the side of a road.
They have charged him with voluntary manslaughter, which carries a maximum sentence of 30 years in prison.
Investigators only linked Lelandais to Noyer’s death after he was arrested over the murder of Maelys, who vanished in the early hours of August 27th, 2017, while attending a wedding near Chambery with her parents.
Police searched for months for the girl before arresting Lelandais, who was also a guest at the wedding, and he finally led them to her remains in February 2018 after traces of her blood were found in his car.
The two cases sparked fears that Lelandais could be involved in dozens of other unsolved disappearances in the region, and reopened several cases after reviewing his background and movements over several years.
Despite three years of inquiries, however, and psychiatric exams that have revealed signs of Lelandais’s “pathological lying,” no evidence has emerged to link Lelandais to other cases.
The trial over Noyer’s killing is set to run until May 12th, while his trial over Maelys’s death is expected next year.