Thousands of Syrians have vanished without a trace, forcibly "disappeared", since the country's uprising began in March 2011. Held incommunicado by the regime or militant groups at secret locations across the country, detainees are often kept in inhuman conditions. Some are tortured, others go on to be killed. For relatives left behind, the pain of not knowing what has happened to their loved ones is intolerable.
The recent beheadings of Western hostages by Islamic State militants in Syria sparked horror and outrage around the world. Yet many more Syrians are also suffering at the hands of kidnappers from violent rebel groups or government security forces - their existence often denied by their captors.
According to human rights groups, thousands of men and women - as well as some children - have been snatched from the country's streets, homes and workplaces before being taken to official or secret detention facilities. They can languish there for years without legal recourse or contact with their families.
Such enforced disappearances and arbitrary detentions have been a feature of the Syrian uprising since it began, says Lama Fakih, Syria and Lebanon researcher for Human Rights Watch (HRW).
But, while in the first instance the target was primarily protesters, Ms Fakih explains, as time went on human rights activists, journalists and lawyers - the legitimate monitors of government activity - also became victims.
Many just disappeared, leaving families with no knowledge of their whereabouts, she says.
"In effect, these people just vanished."
'Infidels, enemies of God'
This is exactly what happened to 53-year-old writer and human rights activist Samira al-Khalil.
Her husband, Yassin al-Hajj Saleh, describes how Samira was snatched along with three of her colleagues - human rights lawyer Razan Zeitouneh, Razan's husband Wa'el Hamadeh and lawyer Nathim Hamadi. They were kidnapped on 9 December 2013, one day before the country's annual Human Rights Day.
A group of masked, armed men stormed Samira's office on the outskirts of Damascus in East Ghouta, says Yassin, capturing the four of them along with their laptops. One of those abducted was on a Skype call with his brother at the time. He overheard the captors shouting "infidels" and "enemies of God" at their victims.
Yassin, also a campaigner and writer, was "in total shock" after hearing the news via phone calls from friends.
"I didn't do anything, I couldn't do anything," he recalls.
The abductors, Yassin believes, were members of the militant Islamist group Jaysh al-Islam, which controlled parts of Damascus at the time. They objected to Razan's plans to set up a judicial organisation in the capital's east, he says.
It represents a long struggle that did not just start today, or with the Syrian revolution, but a long time ago, as part of our struggle with the Syrian regime itself
But, he argues, such arbitrary arrests and prolonged detention without trial are "not new" to his home country.
"It represents a long struggle that did not just start today, or with the beginning of the Syrian revolution, but a long time ago, as part of our struggle with the Syrian regime itself."
Yassin is speaking from experience. Now 53, he was just 19 when he was arrested by the regime for being a member of a communist opposition organisation. He went on to be held for 16 years.
Moved from place to place, it was his last year in prison that turned out to be the worst.
"I was constantly tortured, slapped, hit and lashed," he says. "In some cases, they used hunger as a torture method. Imagine getting four olives for breakfast for months. It was very painful."
For this reason, he fears for the health and safety of his wife and her colleagues.
"We don't know if they have enough food, clothes - if they can see the sunlight, go for walks or are in good health," Yassin says. "It is exactly these things that I experienced myself, along with hundreds of my friends who were detained."
Scale of disappearances
Mass arrests by regime forces, leading to the enforced disappearance of large groups of fighting-age men, have been documented by the UN's Commission of Inquiry on the Syrian Arab Republic - set up in 2011 to record the conflict's war crimes.
Such incommunicado detentions have been employed by the government to "silence the opposition and spread fear amongst relatives and friends of demonstrators, activists and bloggers", the commission says.
The Syrian Network for Human Rights estimates as many as 85,000 people are currently being held arbitrarily by the regime.
Syria's Violation Documentation Center (VDC), which has been recording human rights violations in Syria since April 2011, also believes the number runs into the "dozens of thousands".
On top of this, the VDC has records of more than 1,200 people kidnapped by armed groups - mostly by the jihadist Islamic State (formerly known as Isis). A further 2,600 people are documented as missing without a trace, the VDC says.
However, the UN's human rights office believes the true scale of forced disappearances will only be fully grasped in the aftermath of the conflict.
What are enforced disappearances?
An enforced disappearance is an arrest, detention or abduction, followed by the captors' refusal to acknowledge it occurred. The fate or whereabouts of those "disappeared" is therefore concealed from family and friends. It also places the detainee outside the protection of the law.
Cases of enforced disappearances carried out by the Syrian regime were first documented by the UN's Commission of Inquiry on the Syrian Arab Republic during the pro-democracy protests in March 2011. Numbers escalated as unrest turned into armed conflict. In some instances, the disappearances appeared to have a punitive element, targeting family members of defectors, activists and fighters as well as those believed to be providing medical care to the opposition.
During the last two years a number of anti-government armed groups have adopted similar techniques, abducting civilians perceived to be unsupportive of their cause. While some of these kidnappings can be classed as enforced disappearances, with victims vanishing without a trace, other detainees have been taken hostage - placed under the threat of death until their release is negotiated for ransom or a prisoner exchange.
Source: UN human rights office
Testimony from those who have survived arbitrary detention or forced disappearances has revealed how victims endure desperate conditions with little or no legal representation. Human Rights Watch (HRW) has documented systematic torture at 27 government-run detention facilities across the Syrian regions of Aleppo, Damascus, Daraa, Homs, Idlib and Latakia.
Detainees, questioned by HRW investigators, tell of overcrowded cells where captives are forced to remain standing or take turns sleeping. Many describe being deprived of food, while others recall being kept blindfolded, handcuffed or even naked.
They also describe prolonged beatings, rape, electrocution, mock executions, burning and the extraction of finger and toenails. Some of those held die as a result of their maltreatment.
Three former war crimes prosecutors, who examined 55,000 digital images of dead prisoners taken between March 2011 until August 2013, believe as many as 11,000 detainees could have been systematically tortured and executed since the start of the uprising. The Syrian authorities have denied the claims.
While most of the detentions documented by human rights organisations so far have been carried out by the regime and its agencies, the number of enforced disappearances by armed opposition groups - particularly the Islamic State - are on the rise, the UN says.
Amnesty International has documented cases of torture, flogging and summary killings at IS secret prisons.
'Confusion and chaos'
And for families trying to trace loved ones held by armed opposition groups, such as IS, the search is particularly hard because there are no official channels to explore.
The relatives of photographer Mohammed Nour, who disappeared following a car bombing in Raqqa, northern Syria, in August 2013, faced an agonising struggle to find out whether he was dead or alive.
When the 22-year-old's charred and melted camera was found close to the wreckage of the blast, they had presumed the worst.
Older brother Amer Matar even received a call from a friend informing him of Mohammed's death.
"I was in a total shock," the 28-year-old recalls. "I checked my computer and the news was all over the internet."
The family rushed to the site of the bombing to look for confirmation.
"We searched but we couldn't find his remains or corpse," says Amer. "We went to the hospitals, asked people, even Isis fighters, but we couldn't find anything.
"The whole search was like a maze. Many people gave us either wrong or inaccurate information and that caused a lot of confusion and chaos. They were very difficult times."
But the family then began to hear reports that Mohammed was, in fact, not dead but was being held at a prison run by Islamic State in Raqqa.
"Unfortunately, this is how the families of the detainees in Syria learn about their relatives," says Amer. "You can't bribe Isis for information; they don't even acknowledge that they hold anyone in their prisons."
Amer, who was himself detained twice by government forces in 2011, has since learned his brother was abducted while filming the Islamic State operation to plant the bomb in the car in Raqqa. He has not seen him since.
And without any government or financial backing to fight for Mohammed's release, the family feels powerless.
"Our hands are tied, we can't do anything now," says Amer. "We don't even know why they are keeping all these Syrians in captivity."
Maisa Saleh is facing a similar battle. She is desperate for news of her 25-year-old younger sister, Samar, who was taken from the northern province of Aleppo.
In a bizarre twist of fate, Maisa only learned of Samar's abduction by jihadists just half an hour after she was herself released from government detention.
Maisa logged onto the internet to let her family know of her release and saw the news of her sibling's capture on social media.
"At the beginning, I was so happy that I left the prison and I felt so relieved," recalls the 31-year-old. "Everything changed when I learnt about Samar."
As Maisa had no access to the outside world while she was being held, she had no knowledge of the rise of jihadist groups, such as Islamic State.
"I wondered, who are Isis, why are they doing this? How did they kidnap my sister and how come no one was able to do anything about it? All these questions were coming to my mind, but I had no answers."
Over time Maisa learned that Samar and her fiance Mohammed al-Umor - a 25-year-old journalist - had been kidnapped while filming in Aleppo province.
"Two tinted cars arrived and a number of masked men, speaking with a classical Arabic accent, took Mohammed," she says. "Samar protested, asked them to let him go and asked why they were taking him. So they decided to also take her.
"They pulled her hair, dragged her into a car with Mohammed and drove away."
Maisa knows exactly what life is like in detention. She was arbitrarily arrested by government officials in April 2013 while socialising in a Damascus cafe. Her friends were also detained simply because they were with her at the time. Some are still in prison.
For six-and-a-half months, Maisa's family didn't know whether she was dead or alive.
She describes how she was mistreated - slapped and hit - but because of social media coverage of her arrest, her captors were more lenient.
Her companions were not so lucky.
"Some of my friends were brutally tortured," she says. "One almost died."
We wish someone could just tell us anything about them, whether they are dead or alive. Even if Samar is dead, at least give us her body - let us know. Tell us so that we can rest
Maisa was finally released in November 2013, shortly after her sister's abduction. Then, to add to her family's torment, pro-government forces stormed Maisa's house again earlier this year, detaining her cousin and her friend. Both remain in prison.
"They see it as revenge," Maisa says. "They told my cousin, who was brutally tortured, that she had done nothing and that they were keeping her because of me."
Now living in Turkey and working as a journalist, Maisa has struggled to come to terms with her sister's disappearance and has battled periods of depression.
"Whenever I spoke about her or looked at her photos I started crying," she says. "I felt a major loss and pain."
Maisa and her family have tried everything to find out where Samar and her fiance were taken, but without success.
"Until now we don't have any news about her or Mohammed," says Maisa. "We wish someone could just tell us anything about them, whether they are dead or alive. Even if Samar is dead, at least give us her body - let us know. Tell us so that we can rest."
'Never give up'
UN Security Council Resolution 2139, adopted in February this year, condemns the "arbitrary detention and torture of civilians in Syria... as well as kidnappings and abductions and enforced disappearances" and demands "the immediate cessation of such practices and the release of all persons arbitrarily detained".
Human Rights Watch has also called on the UN to demand access for independent monitors to all detention facilities. The campaign group also wants the situation in Syria referred to the International Criminal Court to ensure there are consequences for the perpetrators.
Since his wife's capture, Yassin al-Hajj Saleh has been campaigning alongside international human rights groups for the release of all those erroneously held in Syria.
He describes how he is connected to all the "mothers, fathers, wives, husbands and siblings" struggling to find out what has happened to their loved ones.
"Mothers say they feel pain every time they talk about their detained or kidnapped sons - when they eat, when they go for a walk or run their usual daily tasks," he says.
"Well, I feel exactly the same. I think of Samira every moment, every second of every day."
And, he adds, he and his fellow campaigners will do "everything" in their power to ensure that those responsible will one day pay for their crimes.
"I will not give up on this cause until I die."
Written by Lucy Rodgers; telephone interviews of Syrian family members and translation by Faisal Irshaid; videos interviews by Andrea Bernardi; design and illustration by Gerry Fletcher; development by Nzar Tofiq.