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 stillnes 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 aftermath of the Arab spring, President Bashar al-Assads regime cracked down heavily on any and all opponent. Demands for reform were met by gunfire and the security forces stimulated 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 recognise 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 dont leave my intellect. 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 prove more than 6,700 corpses of those who died in the custody of the regime. Many of the bodies are emaciated and prove 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. Households 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 appear 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, detecting the image of her son Ayham brought some relief. There was a sticker on his forehead saying he was corpse 320 are subordinate 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 Ayhams photograph some close for Mariam, but she still has no clue where his body is. She dreams of having a tomb 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 assassination, rape, torment 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 tribunal was vetoed by Russia and China.

The Syrian regime has repeatedly rejected 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 their lists 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.

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