And the subject remains touchy here. Many gatherings like the one Mr. Mastura was invited to have been broken up by threats of violence, including several this year in other parts of the country. But he showed up, and he told the small crowd a remarkable story.

Fifty years ago, when he was still in high school, he had been put to work in the anti-Communist sweep. He was assigned to guard prisoners and prevent their escape.

He now says he knows that at least four of the Palu prisoners were later killed. In the years since, he watched as many of the others and their families endured forced labor and unrelenting discrimination.

One of the event’s organizers, Nurlaela A. K. Lamasitudju, sensed an opening. “Is there anything you want to say to the victims’ families?” she asked. Mr. Mastura fumbled a bit, and then apologized.

That had not been the plan. Some people who attended the event said he became emotional and got carried away. Others say it had been the result of a long-term campaign by Ms. Lamasitudju and other rights advocates. Mr. Mastura himself said he had just come to a realization.

The victims and their relatives, he explained in a recent interview, “they did not know anything. But they have to live in conditions that make them have no future, especially their kids, because of political differences. This moved me. That’s when I apologized.”