Valentin Marcone, 29, surrendered peacefully on Friday evening after an 83-hour manhunt in the rugged terrain of the Cevennes region involving 350 police, helicopters, dogs and drones.
He appeared before a judge on Sunday afternoon, but refused to say anything. He remains in police custody.
"Multiple elements" suggested the killings at the sawmill where he worked in the village of Les Plantiers (Gard) were premeditated, Nimes prosecutor Eric Maurel said on Sunday.
The suspect risks life in prison if found guilty for the murder of his boss and a colleague.
Marcone, "was armed with a loaded gun... as he arrived at work and said he had argued with his employer and a colleague that morning about payment for overtime," Maurel said.
Gendarmerie colonel Bertrand Michel said that he may have decided to kill after overhearing "a conversation between his boss and the colleague about firing him for serious misconduct".
Hidden weapons
He "did not express remorse" when arrested and "presented himself as a victim reacting to an attack," Michel added.
Maurel said Marcone had been wearing a bulletproof vest at work for almost three years and going armed for several months, fearing people he "may have clashed with" in the village.
A pistol that Maurel said "could be the murder weapon" and parts of a rifle were found hidden during the chase, with Marcone telling investigators he discarded them as he "didn't want to hurt the gendarmes".
He had immediately admitted to the killings and later repeated the confession, Maurel said, calling him "a calm young man" speaking "logically and coherently".
He had spent his time holed up in a boar's den, surviving on water trickling in a nearby stream but was visibly weakened from lack of food.
The rugged Cevennes, a bastion of French Protestantism, is one of the most remote regions of France.
Numerous French Resistance members hid out in the area's caves and forests during World War II.
