For weeks after the coup, the military rounded up political and social activists and suspected supporters of the former president, Salvador Allende, and brought them to the concrete edifice, which opened in 1938 and hosted matches at the 1962 World Cup, including Brazil’s 3-1 victory over Czechoslovakia in the final.

“I can remember some of the other prisoners talking about going to games there,” Mr. Castro said.

It was the stadium’s intended purpose — international soccer, or at least the prospect of it — that eventually forced the Pinochet government to end its use as a prison camp on Nov. 9, 1973. That month, officials began preparing it for Chile’s scheduled World Cup qualifying match against the Soviet Union. The teams had played a scoreless tie in Moscow in the first leg, but when the Soviets complained about the site of the return match, saying the stadium was a place of blood, FIFA, world soccer’s governing body, said it would investigate.

Many of the prisoners, including Mr. Castro, were rounded up and taken below on the day FIFA officials arrived, into dressing rooms underneath the stadium where they could not be seen from the playing field. At gunpoint, Mr. Castro said, they were instructed to remain silent. But other prisoners were left in the bleachers that day, and remembered watching the men from FIFA go about their inspection.