The Rev. François Ponchaud, a French Catholic priest whose book “Cambodia: Year Zero” alerted the world to the atrocities being committed by the communist Khmer Rouge that would eventually take the lives of nearly 2 million people, died Jan. 17 in Lauris, France. He was 86.

His death was announced by the Paris Foreign Missions Society, of which Ponchaud was a member. The society said he died at its retirement facility. The cause was cancer, a friend, historian Henri Locard, said.

In 1975, at the end of the Indochina war, only sketchy accounts of the Khmer Rouge horrors had reached the outside world, and they were widely dismissed by those in the West who wanted to put the conflicts in Vietnam and Cambodia behind them.

Ponchaud, a priest who had spent a decade in Cambodia and was fluent in the language, was expelled along with other foreigners when the Khmer Rouge took control of the country and sealed its borders.

The Khmer Rouge evacuated the entire capital city of Phnom Penh — a chaotic forced exodus in which thousands died — and for the next four years turned Cambodia into a vast labor camp scattered with torture houses and killing fields, where close to one-fourth of the population were executed or died of starvation and overwork.

After his expulsion, Ponchaud set to work collecting hundreds of written and oral accounts from refugees along the border with Thailand and in France, placing them side by side with information from the propaganda broadcasts of the new government.

His revelations began with articles in the French press that were fiercely attacked by leftists clinging to a romantic view of revolutionaries who had thrown off the yoke of French colonialism.

Ponchaud’s book — sober, detailed and thoroughly documented, with the most horrific scenes told in the words of the refugees themselves — was published in 1977. The catalog of horrors was hard to deny.

“Ponchaud came as an annoyance to people who wanted everything to be lovely in Indochina, with the ‘great new dawn’ and all that nonsense,” David P. Chandler, a leading historian of Cambodia, said in an interview.

“Everyone in the West was fed up with Indochina and wanted to get out of it after 1975, and didn’t want to pay attention to what was happening,” Chandler said. “It was too much to handle in the late ‘70s.”

The phrase “year zero” caught the imagination of the public, although the Khmer Rouge did not use it. It was an apt description, however, of their shutdown of history and culture in an attempt to restart the country and create a pure agrarian utopia.

To achieve this, the Khmer Rouge began by executing high-ranking government and military officials and then moved on to killing educated people who brought the past along with them — teachers, lawyers, Buddhist monks, court dancers — as well as members of ethnic minorities, including Vietnamese and Chinese people and Cham Muslims.

Ponchaud witnessed the evacuation of Phnom Penh, in which even hospital patients, some wheeled in their beds, were forced into the streets.

“I saw the unspeakable event,” he testified at a United Nations-sponsored trial of Khmer Rouge leaders in 2013. “I saw sick people, I saw the crippled, who were crawling like worms right in front of my house.”

The purges spread through the mostly rural nation, as low-ranking members of the Khmer Rouge rooted out people they considered tainted by the past and executed those who disobeyed them. Slaughter — or “self-slaughter,” in Ponchaud’s words — eventually became the defining characteristic of the regime.

“Unending labor, too little food, wretched sanitary conditions, terror and summary executions: From these, the hair-raising human cost of the Khmer revolution can be imagined without much difficulty,” he told the French news organization Agence France-Presse in 2021.

In one of the book’s many descriptions of the terror that permeated the country, a woman told of climbing a tree when she heard the Khmer Rouge approaching, at the risk of having her legs eaten by red ants, as “some children were being torn apart and some were being impaled.”

The book quotes a refugee who said he worked in a hospital with 300 beds. “It was a hospital in name only because sick people were sent here so that their families wouldn’t waste time looking after them instead of working,” the witness said. “Large numbers of people died every day. The 20 or 30 people running the hospital both cooked the rice and carried away the corpses.”

François Ponchaud was born on Feb. 8, 1939, in Sallanches, a small village in the French Alps.

He worked with his parents on their farm, he said, until he was 20. “We had 12 cows, pigs and chickens; we had a lot of fruits and berries — strawberries, apples and pears,” he told The Phnom Penh Post in 2013. “I have six brothers and six sisters, but I’m the only one who became a priest.”

In 1959, after a year in a seminary, he was sent to fight in the separatist war in Algeria, serving for 2 1/2 years. He took parachute training but said he never jumped in combat.