The body of a former French intelligence agent linked to an assassination plot has been found riddled with bullets in a layby in the Alps.

Daniel Forestier was discovered in a pool of blood in a parking area off a little-used road in Haute-Savoie near Lake Léman. Police said the killing was a “professional job”, and a postmortem revealed he had been shot five times.

The killing of Forestier, 57, author of several spy novels, came six months after he had been officially put under investigation for his suspected role in a plot to kill an opponent of the regime of Congo-Brazzaville’s president, Denis Sassou-Nguesso.

Police said Forestier had previously served for 14 years as a member of France’s counter-espionage agency, the DGSE, the French equivalent of MI6, but did not give dates when he was an operative. He was reported to have been a member of a special military unit tasked with top-secret “sensitive operations”.

France Bleu radio in Haute-Savoie said Forestier lived in the village of Lucinges, nine miles (15km) from where his body was found, with his wife and two children and had served as a local councillor until he was put under investigation last September.

The public prosecutor said the killing was probably a settling of scores. “There’s almost no doubt about it,” Philippe Toccanier said. Forestier had been shot five times in the thorax and the head, he added.

The Lucinges mayor said everyone in the village of 1,640 residents knew Forestier had worked for the country’s intelligence service.

“He’d written several spy novels, but he never gave us any details of what he did,” Jean-Luc Soulat told the local radio station. “He was very well settled here. He ran a bar-tobacconist here and only 15 days ago he helped me organise the opening of a village hall.”

He said when Forestier resigned from the local council last September he said it was for “personal reasons that he would explain one day. Now I understand why he resigned and I think it was because he wanted to protect the community.”

In a plot worthy of a John Le Carré novel, Forestier and a second former intelligence operative were put under investigation for allegedly planning to kill Gen Ferdinand Mbaou, a senior opposition figure in Congo-Brazzaville. Both were indicted by an investigating magistrate last autumn for “participating in a criminal organisation” and “holding explosives” .

Le Monde reported that Forestier had admitted being part of a group formed to kill Mbaou, who was living in Val d’Oise, north of Paris, but told investigators after carrying out reconnaissance he realised the plot was “not possible”. The paper said Forestier later denied being involved in any criminal activity.

Mbaou, 62, was head of the presidential guard to Sassou-Nguesso’s predecessor, Pascal Lissouba, but fled the country when Lissouba was overthrown in a coup in 1997. He has been a fierce critic of the Sassou-Nguesso regime since and still has a bullet lodged near his heart from a previous assassination attempt. He told Paris Match last year he only learned of the plot to kill him when he read about it in the French newspapers.

“I know why they want to kill me. I was warned and also I received threats in text messages. I tried to warn the [security] services but they didn’t do anything,” Mbaou said.