To this day, there is fevered speculation about what was done to the bodies. Some people said they saw trucks leaving the prison soon afterward, and heard that they had gone to local graveyards, to dispose of the bodies. Other people said the security forces buried them on the grounds, then moved them outside the walls, because of the smell. “It was like living behind a secret,” Mr. Rayes, the neighbor, said.

Faraj al-Gorgi’s family heard rumors about the shooting from prison guards, but did not know his fate. To this day, they have never been told the truth. In 2002, security men delivered a death certificate that said Mr. Gorgi had died the year before. No cause of death was included, but a security officer told the family that Faraj had been ill. The family was not told where the body was, but was asked not to erect a large mourning tent, as is the custom. A government minister visited the family and asked them to take compensation and stop asking questions. Mr. Gorgi’s brother, Jamal, said they refused.

In Benghazi, many refused, and a few years ago, the families there started holding protests on Saturdays. In February, after revolutions in Egypt and Tunisia had toppled their rulers, the Libyan authorities, nervous about any unrest, arrested Fathi Terbil, a lawyer who represented the Abu Salim families and who had three relatives killed in the massacre. His clients gathered outside the security building where he was being held, and refused to leave until he did, said Mr. Bensoud, whose brother, Abdullah, had been killed in the 1996 massacre.

“We decided we had to make it burn,” he said.

The protests began to draw people with many grievances. Demonstrators started calling for the downfall of the government. Security forces attacked, the crowds swelled, and one week later Benghazi became home to a revolution. The security forces withdrew.

In decades past, Abu Salim was filled mostly with prisoners from eastern Libyan towns like Benghazi, the seat of the opposition, or Darnah, the country’s most pious city. Some were Islamist militants with experience fighting in Afghanistan, and some were picked up simply because they had long beards. The government referred to the men as “heretics.” Secular opponents were also held at the prison and tortured. Many of the men were held without trial and not allowed visits from their families.

As the protests grew into an uprising, the cells were jammed with hundreds of men swept up by the security forces, including protesters or even others who happened to walk by a demonstration. Last week, after Colonel Qaddafi fled, the open cells at Abu Salim began to reveal their secrets. Histories were written in graffiti on the walls, or on the covers of boxes in which prison food was delivered.