Here Gaddafi's men trapped their prey – then threw in the grenades

Amr Dau Algala was picking through the ashes with a stick when he came across the charred and broken bones. A little later he found the buckle. "Only my brother was wearing a belt in our group. This looks like my brother's," he whispered, looking down at the twisted piece of metal.

Around 60 men, prisoners of Muammar Gaddafi's regime, died when guards first opened fire and then tossed grenades into the warehouse where they were being held. Among them was Amr and three of his brothers. They ran for their lives amid the flames, noise and confusion and escaped. A fourth, 25-year-old Abdullah, is missing.

"The last time I saw Abdullah was there, sitting in that corner," said Mr Algala, pointing at a blackened corner of the metal box, around 25 feet long and 20 feet wide, into which more than a hundred captives had been crammed. "He is young and looked very scared that morning. When the guards opened fire I started running. I looked back, but there was too much smoke, I could not see my brother. Some people got away after us, we are really hoping Abdullah was one of them, but we don't know."

Six mounds of reddish brown earth, in a stretch of ground next to the headquarters in Yarmuk of the 32nd Brigade, commanded by Khamis, Colonel Gaddafi's son, marked where the remains of the prisoners had been buried. Decomposed by the heat in the shallow graves most of them have been impossible to identify.

The revolutionaries in Libya claim that up to 50,000 people have died or disappeared in the hands of the regime's forces since the uprising began in February. Caution is needed about such numbers and the figure may well be too high. But it is also the case that what happened to the Algala brothers is just one of many examples of Gaddafi's forces taking vicious retribution in the dying days of the regime.

Abdul Birbash was sitting outside the brigade compound, at Gasar ben Ghasir, a suburb of Tripoli, in an old Toyota car clutching photocopied images of his brother, Abdullah, 31, cousin Hasaib, 24, and 26-year-old Salah Nouer, a neighbour. They had disappeared returning from the town of Zintan on the night of 23 August.

"They were bringing back some people who had been arrested by Gaddafi, but there were still government soldiers around and they must have run into them. They were with another man whose body has been found, he was shot and burned," said Mr Birbash, 21, a student.

"We have driven hundreds of miles trying to find out what has happened, I don't think they are here, the dates do not match. My mother, my aunt, are very worried. We do not know what to tell them."

Amr Dau Algala, 34, knew that he would receive particularly harsh treatment from the regime because he had joined the underground opposition in the Libyan capital while still a serving policeman.

He had done his bit to sabotage the machinery of state, he said, by destroying messages from embassies abroad he had been tasked to decode. But with Colonel Gaddafi clinging on to Tripoli, Mr Algala decided to play a more active part and started smuggling guns into the city.

"There were many, many police officers who were working in secret to bring down Gaddafi. But someone must have said something and there was a raid at night. They took me and my brothers away," Mr Algala recalled.

The Algalas were taken to Abu Salim prison, a place of fear where 1,200 inmates were slaughtered following riots in 1996. "We were beaten repeatedly there with sticks and pieces of hosepipe. They didn't even bother to question us much, they just kept hitting us," said Mr Algala. "After two days we were driven out of Abu Salim. I was put in the boot of the car and I wished that I would die there. I did not want to be taken out and tortured again. My hands were tied with wire, I could not move them, they swelled up."

Mr Algala and his brothers, among others, were locked up for two days and nights in a prison van at the "Khamis Brigade" headquarters. "I don't know how we survived that. But we were taken out and the beatings began immediately. Then we were put on chairs and given electric shocks. My whole body shook. I have never felt such pain. After that we were just thrown into the room and left there."

The maltreatment started again soon afterwards. One warehouse, with bloodstains on the walls and ceiling, coils of orange and green rope on the floor, was where prisoners had been strung up by their wrists. "They were begging to be cut down, but the guards would not listen," said Mr Algala. "They really had no pity. One man had been shot in the leg. He was so thirsty that he drank his own piss. But still they would not give him any water. I believe he died before the guards started shooting."

Mr Algala recalled that one day the guards announced that Khamis al-Gaddafi was arriving himself and the prisoners would be free. "People got very excited and the guards started laughing. They said that being 'free' of this place meant that we will all be killed. We did not know whether to believe them or not."

The threat proved to be real the following morning when the murders began. "Three guards came to the doorway and started firing, they took turns to fire, then there were loud bangs. I realised they were throwing in grenades. We were all shouting and we ran out," said Mr Algala. "We climbed over that wall and we ran through the houses. They came after us, firing, and I saw some people fall. But others got away.

"Maybe my brother was one of them. Maybe he was injured and someone is looking after him. Maybe we shall hear from him soon."