In 1973 the academic and historian Stephen Heder, then a recent graduate, came to Phnom Penh hoping to make his name in journalism, he told the tribunal.

“There was a widespread expectation, after the US ended bombing, the US would withdraw support from Phnom Penh and the Lon Nol regime would collapse,” he said.

“A lot of veteran journalists were waiting around for the collapse. And then the Khmer Rouge didn’t come and they left.”

Heder stepped in to fill the gap and soon was regularly reporting for outlets such as Time Magazine, whose editor asked him for a story about whether Phnom Penh residents thought there would be a bloodbath once the Khmer Rouge won the war.

“For the most part, people said ‘no’,” Heder said.

Good faith in leaders such as Khieu Samphan led most residents Heder interviewed to believe the Khmer Rouge would “do well by the population”.

“There was an alternative view that they were different but worse,” he recalled. Time didn’t publish the story.

Prosecutor Keith Raynor asked Heder to describe what he witnessed before the Khmer Rouge victory in Phnom Penh. “Bring it to life, please,” he said.

“You sound like my editors,” Heder quipped. “It was certainly scary . . . The shelling was indiscriminate.” One volley fell into a neighbourhood to the west of town where lots of people sold gasoline and the stores exploded in flames.

The students and the middle class, such as it was, were fed up with the Lon Nol regime, Heder recalled. But they were also among the groups the Khmer Rouge targeted, according to people Heder interviewed after the Khmer Rouge fell in 1979, he confirmed at another point. In 1980 Heder interviewed Khieu Samphan near the Thai border and asked him whether some people “might have been wrongly killed”.

“No, none,” was Samphan’s answer, although the deposed head of state later allowed that “one old man” in the country’s west had been wrongly accused.

“I knew I had, in a sense, ambushed the man. I don’t think he was prepared for this line of questions,” Heder said. “It was confrontational, it was emotional, on both sides.”

Like the day before, the defence objected several times, accusing Heder of offering his opinions as an expert after having declined to testify as an expert witness.