Desperate Syrians, clinging to the dream of seeing missing sons, husbands and sisters again, say they were systematically shaken down for bribes that together amounted to hundreds of millions of dollars. And, worse, in many cases the assorted officials, lawyers, grifters and Assad clan hangers-on demanding the cash failed to deliver news of the detainees, many tens of thousands of whom are now dead, rights monitors say.

The Association of Detainees and Missing Persons at Saydnaya prison carried out hundreds of interviews to ask how much families had paid in return for the promise of information, a visit or a release from jail. Based on its data, the association estimated that government officials and supporters had made almost $900 million. Hundreds of thousands of people have been detained since protests erupted against Assad’s rule in early 2011. Now, 13 years later, the gates of Saydnaya Prison, a grim, grey-walled complex squatting over an arid valley dotted with plush villas 30 kilometres (18 miles) north of Damascus, hang open.

Now awed civilians and armed opposition fighters wander Saydnaya’s cell-lined concrete halls, kicking over the filthy abandoned sleeping mats that show inmates were packed 20 to a cell. Rescuers have punched holes in walls to investigate rumours of secret levels housing missing prisoners, but many thousands of families are disappointed – their relatives are probably dead and may never be found.

On the ground floor of one wing, fighters and visitors pause in front of a hydraulic press that former detainees say was used to crush prisoners during torture sessions. The floor of a neighbouring room, with more industrial equipment, is slick with foul-smelling grease.

Sanaa Omar, a 38-year-old woman from the northern city of Aleppo, came to the capital Damascus seeking news of her brother Mohammed, who went missing when he was 15. “My brother has been missing since 2011,” she told AFP at a city hospital morgue where opposition fighters had deposited unidentified corpses found in Damascus prisons. "We looked in all of the prisons in Aleppo, in all of the branches. We paid everyone: lawyers would promise us they knew where he was and said they would bring documents, but they never did. “My dad would go every year to Damascus and meet with lawyers or people who would say they work with the government. They would take 200,000 or 300,000 or 400,000 (Syrian pounds) and we’d pay them,” she said. “They’d say: ‘You’ll see him in month’. We’d wait for one month, two months, three months… but they never brought us a visitor’s pass. We paid them for nearly five years, but in the end we lost hope.”

Ayoush Hassan, 66, came from the Aleppo countryside to find her son. “A month ago I paid 300,000 Syrian pounds for them to check his record, and they said he is in Saydnaya and is in good health,” she told AFP outside the jail, her anger rising as despair gripped her. “Not here. Not here. He’s not with us!” she cried, describing how she had found court records burnt, and as a crowd gathered to hear of her grief. “We want our children, alive, dead, burned, ashes, buried in mass graves… just tell us,” she said. “They lied to us. We’ve been living on hope. We’ve been living on hope for 13 years, thinking he’ll get out this month, in the next two months or this year or on Mother’s Day… it’s all lies.”