“I still remember their last words to me: ‘Please don’t forget us’. This rings in my ears every day like church bells, like a daily call for prayer.”

Mansour al-Omari, a Syrian human rights activist, recalls the moment his name was called by the jailer after spending nine months in detention. He was lucky to be released, but is haunted by those he left behind.

Conditions in the Syrian regime’s detention centres are hellish. Detainees describe being held in overcrowded cells, suffering from malnutrition and regular physical and psychological abuse. Thousands have died under torture, or due to the hostile conditions and neglect. And many former detainees, ones I have interviewed for the film Syria’s Disappeared: The Case Against Assad, harbour survivor’s guilt.

Mazen Alhummada, a leftwing activist and employee of an oil company who was detained for 18 months, told us: “When we were imprisoned, we promised each other that if one of us got out we would tell the world what was happening inside. I am determined to expose this regime, just as we agreed. It’s my duty to the people who are still there.” Mansour echoed this: “It is always a cure to my soul’s pain to help those who are still underground.”

Mansour al-Omari: ‘It is our duty to act.’ Photograph: Channel 4

Tens of thousands of Syrians are currently “missing in detention”, a hidden horror of the six-year-old Syrian civil war. Detention has long been a tool of repression for the state to silence and punish its critics, but it has never before been meted out as punishment on such a vast scale.

From the outbreak of the peaceful protests in 2011 in the wake of the Arab spring, President Bashar al-Assad’s regime cracked down heavily on any and all opposition. Demands for reform were met by gunfire and the security forces made mass arrests. With the country since descended into a brutal civil war, the arrests and detentions have continued to the present day.

The Syrian regime refuses to disclose the names of those detained or acknowledge how many people are being held in its clandestine prisons. For families and friends of those detained, this is another form of torture. They search for news about their disappeared loved ones, not knowing whether they are dead or alive.

Mazen has several close family members currently disappeared in detention. “I miss them so much. They don’t leave my mind. I look at their photos every day and they give me strength to keep going.”

Documentation created by the Syrian regime attests to their brutality. Photographs taken by the military police catalogue the dead; thousands of these photographs were smuggled out of the country in 2013 by a defector codenamed Caesar. They show more than 6,700 corpses of those who died in the custody of the regime. Many of the bodies are emaciated and show clear signs of torture, with bruises, burns and eyes gouged out. The corpses are numbered and pictured with a card recording their detention facility.

The Syrian Association for Missing and Conscience Detainees has published headshots of these corpses online to enable identification. Families trawl through the Caesar shots looking for missing loved ones. This grim search is plagued with uncertainty; some faces are mutilated or altered by dramatic weight loss. Relatives look again and again at lifeless bodies trying to recognise people they once knew.

Despite the difficulties, hundreds of individuals have been identified from the Caesar photos. For Mariam Hallak, finding the image of her son Ayham brought some relief. There was a sticker on his forehead saying he was “corpse 320 belonging to detention facility 215”. For Mariam, he was her youngest son, 25 years old, a popular young man who had been studying for a masters in dentistry.

Seeing Ayham’s photograph provided some closure for Mariam, but she still has no clue where his body is. She dreams of having a grave for her son. She also wants President Assad and the heads of the security branches to be prosecuted.

The UN has accused the Syrian government of the murder, rape, torture and extermination of detainees, yet action on accountability for these crimes against humanity has been hampered. A security council resolution to refer Syria to the international criminal court was vetoed by Russia and China.

The Syrian regime has repeatedly refused access to independent international monitors to inspect its detention facilities. Amnesty and other groups have been calling for action on this, and for the regime to publish the names of detainees, their whereabouts and what has happened to the bodies of those who have died.

“We have the evidence,” Mansour pleads. “And there is an urgent need to save those who are still alive. It is our duty to act.”

Syria’s Disappeared: The Case Against Assad is on Channel 4, Thursday 23 March, 10pm.

