The families of hundreds of civilians who were kidnapped by Islamic State and held in its notorious jails have urged the military factions that ousted the group from Raqqa, its de facto capital, to help them find their loved ones.

Relatives say hundreds and possibly thousands of people detained during Isis’s reign of terror in Syria remain unaccounted for despite the group’s loss of territory and retreat to desert hideouts.

“We have hundreds of names, pictures, and arrest dates. There is no full list but we are discovering new names every day,” said Amer Matar, a documentary filmmaker from Raqqa who lives in Germany and started an online campaign to raise awareness about the detainees. Matar’s brother, Muhammad Nour Matar, was detained by Isis in August 2013 and remains missing.

Facebook Twitter Pinterest Poster from the online campaign started by Amer Matar

“None of the military or judicial organisations have dealt seriously with the issue of those kidnapped by Daesh [Isis], and we have had no responsiveness from them,” he said. “Our families are second-class citizens to everyone concerned.”

Isis’s self-proclaimed caliphate has collapsed under a multi-pronged assault in Iraq and Syria by Turkish-backed rebels, US-led coalition forces, Kurdish paramilitaries and the Iraqi and Syrian armies. But few have sought answers as to the fate of thousands of people imprisoned by the group during its more than three years in control of vast stretches of land in the two countries.

The militants have claimed several high-profile prisoners, including journalists who were executed on camera, and Father Paolo Dall’Oglio, a dissident Jesuit priest from Italy who was living in Syria when he was kidnapped. His fate remains unknown.

Many Syrians were also detained on a daily basis for infractions ranging from smoking to protesting the militants’ actions, documenting abuses and “apostasy”.

Matar has teamed up with a network of activists on the ground who have visited abandoned Isis prisons to photograph them and gather any documentation that could cast light on the fate of the prisoners. Some detention centres have already been converted into holding cells by the conquering forces.

Among the team’s findings were the memoirs of an Isis jailer, verdicts handed down by Isis’s courts – including an execution order against a nurse who allegedly confessed to cursing God – etchings by prisoners on the walls, and abandoned clothes and orange jumpsuits that they surmised were removed before Isis killed prisoners.

Facebook Twitter Pinterest The death sentence of a female medic called Fatima Majd Jassem, handed down because she allegedly cursed God and committed apostasy.

The Syrian Network for Human Rights (SNHR) has documented the names of 8,119 individuals who remain missing, including 286 children and more than 300 women. Isis operated at least 54 detention centres at the height of its power, with many more secret prisons and cells, according to the rights group.

A spokesman for the network told the Guardian that at least 1,600 individuals had died inside Isis prisons. The deaths were attributed to a number of causes, including torture, mass execution orders, the targeting of prisons by the US-led coalition during territorial retreats by Isis, and as a result of forced menial labour carried out near the frontlines as punishment.

Facebook Twitter Pinterest An etching dated 8 August 2015 found on a prison wall. It names three people, Youssef (born 1998), Rami (born 1995), and Ahmed (born 1995), and states: ‘We were arrested in the town of Tabqa. Accusations: we are apostates, theft from an Islamic State branch, and use of pills and hash. Thanks to God, we have repented’.

The responsibility of verifying what happened to the prisoners falls on the Syrian Democratic Forces (SDF), an alliance led by Kurdish militias and backed by the US, which took control of Raqqa, the spokesman said.

“The SDF is responsible for uncovering the fate of thousands of forcibly disappeared in Daesh prisons, because it controls broad swaths of areas that were under the militants’ control including Raqqa city, which held the most important administrative centres for the group and its leaders,” he said. “They have also detained dozens of security officials and leaders in the group who definitely have information that would clarify the fate of the forcibly disappeared.”

The SDF did not respond to a request for comment about its efforts to trace former prisoners.

Facebook Twitter Pinterest Prison wall etching by a man who had been imprisoned for 65 days, seemingly written on 29 January 2016. Next to the word ‘exit’ he has written ‘God knows’. Written below the dates is a series of prayers

The families, while sometimes contemplating the worst, still hold out hope. Among them is Zubayda Ismail, the wife of a surgeon from Raqqa who was kidnapped by armed men wearing balaclavas in November 2013. Ismail’s protestations to Isis led nowhere, as the militants denied they had detained her husband.

After fleeing to Turkey and then France, Ismail has posted a note about her husband on the campaign’s Facebook page every day, updating the number of days since his disappearance.

Matar also hopes to one day be reunited with his brother, and wants to open a museum to show people the horrors of Isis’s prisons. “The evidence of these crimes is being eradicated,” he says. “That’s why I thought of preserving this memory that nearly destroyed our lives.”

Facebook Twitter Pinterest Etching that reads: ‘I miss my mother. Three months’

Matar’s brother was detained more than four years ago after filming a women’s protest in front of an Isis building in Raqqa. Despite the lack of knowledge about his brother’s whereabouts, Matar is consoled by the fact that there are many more prisons that have yet to be searched.

“There is great hope,” he says. “Every day we have hope that Muhammad Nour will be back, and we will see him, and meet him. We will find him. We started this campaign for Mohammad Nour and the thousands like him.”