Daniel Forestier, 57, who lived in the Alps region in eastern France, was found dead last Thursday in a remote car park in the small town of Ballaison, near lake Geneva.
He had been shot five times, including in the head and heart, his lawyer Cedric Huissoud told AFP.
Forestier and another former agent from France's external intelligence service, the DGSE, were charged last September with “criminal conspiracy” and “possession of explosives” in connection with a plot to murder the Congolese general Ferdinand Mbaou, who has lived in exile in France for some 20 years.
Congolese general Ferdinand Mbaou has lived in exile in France for 20 years. Photo: AFP
Forestier is believed to have worked for 14 years for the DGSE. He was living in the small town of Lucinges, near the Swiss border, where he had served on the town council and ran a cafe.
In October, Mbaou told AFP he was angry at the reported plot, but “not surprised”.
Like a number of France-based opponents of Republic of Congo's President Denis Sassou Nguesso, Mbaou believes he was targeted for criticising one of Africa's longest-serving leaders from what he thought to be a safe distance.
The 62-year-old general is known for his outspoken attacks on Sassou Nguesso, who has ruled the former French colony and oil-rich central African country of 4.5 million people for some 35 years.
Mbaou fled Congo after his former boss, the country's first democratically-elected president Pascal Lissouba, was overthrown by Sassou Nguesso in 1997.
He had already survived an attempt on his life.
Mbaou believes it was the regime that sent hitmen to shoot him in the back as he was leaving his home in Bessancourt north of Paris in November 2015.
He still has the bullet lodged in his torso.
“The doctors couldn't remove it because it is in a tricky spot, close to the heart,” he told AFP.
No one was ever charged over the attack.