On the following Friday, Marwan’s father returned, this time with two relatives. Nasser helped them carry crates of food — yogurt, fruit, homebaked biscuits — down to Marwan’s cell. When Nasser came back upstairs, Marwan’s father was standing by the door. He went straight up to Nasser and looked him sorrowfully in the eye. “He embraced me and kissed me on the forehead,” Nasser said. “So he must know.”

Two days later, as we talked in his office, Nasser asked me: “What’s the definition of revenge? To make the family of the person who did it feel what my family felt? I could have killed Marwan at any time, nobody would have known. But I don’t want to betray the blood of our martyrs. We want a country of laws.” He picked up the files on his desk and put them into his cabinet. He seemed preoccupied, as though he were trying to convince himself of something. He rubbed out his cigarette in an ashtray and turned to me again. “Besides,” he said, “where is the honor in taking revenge on a prisoner?”

I couldn’t be sure exactly what was motivating Nasser in his long struggle with Marwan. Certainly part of it was anger, which has not subsided and possibly never will. But the long months of interrogations had given him an unexpected solace, too, a chance to get to know his brother better and to sift through his own failings. “I keep asking the prisoners small details, like how many times he was beaten, what he talked about, how he seemed,” Nasser told me. “How he used to get into fights, demanding proper medical attention for the other inmates. Whenever they were tortured, they would be brought to his cell so he could treat them.” Nasser had been moved by the stories he heard of his brother’s bravery. Once, Omar paid a guard to take a prescription notice to a pharmacy. He had written a plea for help on the note, in English. But the woman at the pharmacy simply translated the note for the guard, who went straight back to Yarmouk and beat Omar severely. Omar kept on trying, sending notes to colleagues who either could not, or would not, help.

One thing in particular was haunting Nasser. According to the prisoners, Omar had talked a lot about Nasser in jail, saying he was sure his brother would rescue him if he could. “I feel such remorse I wasn’t able to help him,” Nasser said again and again. He told a long story about a well-connected soldier he’d known, who might have been able to do something if he had pushed him hard enough. He said he hadn’t seen Omar during the last days before the arrest, and now he chastised himself, imagining alternative endings. “I would’ve done anything, even gone to the front for Qaddafi’s people, if that would have saved my brother,” Nasser told me. “At the end of the day, it’s what’s inside you that counts.” But he didn’t sound convinced.

Nasser didn’t stop with the recent past. He reviewed his whole life for me, trying to understand where he went wrong. He was always the family’s bad angel, he said, a prodigal son. Omar was the conscientious one. He returned to Libya after a decade abroad in 2009, telling friends that he was ashamed of Libya’s backwardness and eager to help out. He brought back books about Qaddafi written by dissidents and a conviction that the country needed to change. At the time, Nasser told me, he thought his brother was being naїve. Now he understood he was right. It was as if Omar had become a screen onto which Nasser’s own failures were projected: the lies, the cowardly survival mechanisms that come with living under a dictatorship. I had the sense that Nasser was struggling to learn from his brother, and in an odd way, trying in turn to teach something to Marwan. After Marwan’s family left, Nasser went downstairs and spoke to him. “I said, ‘Look what I did, and look what you did,’ ” Nasser told me. “ ‘You killed my brother, and I arranged for you to see your family.’ ”

Omar’s life cast a similar shadow onto other people. One was his closest colleague, a doctor named Mahfoud Ghaddour. Omar’s fellow prisoners from Yarmouk told me he was always trying to contact Ghaddour, whom he saw as a possible savior. In fact, Ghaddour was aware that Omar was being held in Yarmouk — one of the frantic messages Omar sent from the prison got through to him — and yet he did nothing. Ghaddour told me so himself, during a long talk in his office at the hospital. “I started looking in that place,” he said, “using contacts with people in the government. But it was somewhat difficult. They started changing their mobile phones. I had difficulty getting help.”

Ghaddour said this with a wincing half smile. I found it impossible to believe. I knew other people who got relatives out of Yarmouk. As a prominent doctor, Ghaddour had plenty of contacts he could have called on. And even if he failed, he could at least have told Omar’s family, or his in-laws, who were desperate to know where he was being held. Ghaddour must have sensed my skepticism. He continued with a long, rambling narrative in which he tried to blame other people for not rescuing Omar from the prison and talked at length about how dangerous it was in Tripoli at that time. But there was something pained and apologetic about his manner, as if he were groping toward a confession. He cared about Omar but did not want to make trouble for his own family. He had done what so many others had done in Qaddafi’s Libya — kept his head down and let others take the risks. These are the survivors in Libya, the ones who adapted to a place where fear was the only law. Most of the brave ones are dead.