It has been more than two months since the US-backed Syrian Democratic Forces pushed ISIL out of its former capital Raqqa and other strongholds in Syria.
But the fate of thousands of men, women and children held by the armed group remains unknown.
The Islamic State of Iraq and the Levant (ISIL, also known as ISIS) operated an estimated 54 detention centres, as well as several secret prisons and interrogation facilities across Syria, with at least 7,419 detainees, according to figures released by the Syrian Network for Human Rights.
Family members and activists are calling on the international community to find out what happened to those held in ISIL prisons in hope of some closure, and to demand accountability for human rights abuses by the armed group's fighters.
As 2017 - which saw the destruction of ISIL's so-called caliphate - draws to a close, Al Jazeera brings the painful accounts of Amer Matar and Hiba Alhamed, who both experienced the abduction of a loved one by ISIL.
Amer, 31, now lives as a refugee in Germany. He addressed the following letter to his brother, Mohammed Nour - an opposition media activist - who disappeared after an ISIL car bomb attack near Raqqa's train station on August 13, 2013:
My little brother Mohammed Nour, I miss you.
It's been 1,597 days and I haven't heard anything from you.
I've looked for you in half of the prisons that Daesh [Arabic acronym for ISIL] left behind but to no avail. I didn't find your name scribbled on the walls nor within the inmates' files or in the pages of the prisoners' diaries.
For years I have been writing about you, but today I have decided to write to you. The hope of seeing you one day rises and falls at an alarming pace after the world destroyed our city Raqqa and Daesh left it.
Do you remember our home, our neighbourhood and the Rashid garden? Everything has now turned into burned concrete.
Mohammed Nour, the night you disappeared was the most painful night of my life.
I was on my way from Munich to Raqqa. That day, I was late for my plane in Ankara and the next flight to Urfa [city on the Syrian-Turkish border] was not until the next morning.
I woke up in the middle of the night in a hotel room inside the airport and opened my Facebook account. I saw a message from a friend from Raqqa telling me that you were missing, along with several others, after Daesh detonated a booby-trapped car near the train station during its battle against the Free Syrian Army.
Our older brother Mazer was just a few metres away from the explosion. He had to wait until morning to identify the charred corpse that was under the car, to see if it was yours.
On my way to the airport, I could do nothing but cry. One corpse was lying there all night, no one could reach it because Daesh had imposed a curfew on ambulances. One corpse. If it was not yours Mohammed Nour, then it was someone else's brother.
I was trying to keep myself from crying while boarding the plane. I called our brother 1,000 times, but the connection was bad. I was desperate for news on the body that was trapped under the car.
A few hours later, I reached Raqqa. All the windows of our house were wide open. I went upstairs and our aunt opened the door. She screamed like a lunatic: "Mohammed Nour, Mohammed Nour…"
Her voice terrified me. I froze. Our mother came running towards me. She hugged me and told our aunt: "This is Amer not Mohammed Nour."
The only thing we could find from the car explosion was your charred camera. We thought you died that night, but we searched for you for days among the dead and the injured.
We then received confirmation information from activists and released prisoners that Daesh had kidnapped you while you were trying to escape from the scene, and that you were being held in one of their prisons.
We tried to extract videos from your burned camera. We succeeded after months of trying but what we found was not helpful. In our efforts to look for you, we experienced many instances of doubt, deception, fraud and blackmail, but we remain certain you will return.
You will come out and you will be surprised to see our little brother Hazem's features. He was a child when you disappeared, today he is almost your height. Our mother has also started looking older.
Our lives have completely changed and the way we see every detail of the house, food and life have also changed. Mother did not close the windows of our house in Raqqa for a whole year, waiting for you to return. She now keeps fake flowers on the balcony of her small home, south of Germany, because she is tired of death.
Mohammed Nour, a few months ago, we silently remembered your 24th birthday. I don't know why I am writing to you these few sentences in plural. Maybe because this grief I feel while writing this letter sounds like the voices of mum and dad when they talk about you.
Years are passing, Mohammed Nour, and the regime of [Syrian President Bashar] al-Assad has not fallen. We did not get freedom in Syria - just death, destruction and disappointment.
The film that you were shooting about Raqqa's fight against the dictator Assad, and the transition of the battle into one against extremism and Daesh, is now about your absence. It is about the search for you in tens of Daesh's prisons that we managed to enter, with the hope of finding any thread that could lead us to you.
Mohammed Nour, I hope we meet again in the near future. I hope to walk with you, whether in exile or between the rubbles of our destroyed city.
Now living in France, 23-year-old Hiba writes to her father Ismaeel Alhamed, a Syrian doctor and opposition political activist, who was kidnapped and held by ISIL in Raqqa, on November 2, 2013:
Dear father,
I still remember the day when you told me, almost in jest, that you had spent your first four years in medical school wearing the same shirt and pair of pants, because you had just enough money to pay for a room in the university dorm in Aleppo and a little bit of food.
I hid in my room and cried that day. My heart could not bear to fathom the conditions you had lived in especially among medical students who for the most part belong to Aleppo's rich, upper and middle class.
What you had told me was something for us to take pride in. You managed to build a life from scratch and become successful. But as a young girl who was barely 10 years old at the time, I could not grasp the idea that my beautiful, gentle and strong father had suffered in that way.
I used to think that that period of your life was the most difficult one. But now, as I recount those memories, I realise how naive I had been and how life had deceived me. I admit that you went through some tough times but it doesn't compare to now when I imagine you in Daesh's prison - considered to be one of the most dangerous in the world.
How am I supposed to spend my years studying in France where I came as a refugee, when despite having a closet full of clothes - a veil of guilt covers my face each time.
Those monsters kidnapped you and took you away from us. It was my first day in medical school, the one where you went. Daesh wanted your absence to be my burden of suffering through my years as a student.
Your long absence with all the pain it inflicts never ceases to bleed our tired hearts. Daesh wanted you to go through an even harsher and crueller experience - enforced disappearance at their hands as the new oppressors of Raqqa.
The oppressors who deliberately ignored your bright smile filled with love and humanity. They did not care that your clinic had shut down after you were kidnapped - the same clinic that secretly welcomed the injured who suffered the regime's brutality during demonstrations, those who the Assad regime wanted to kill because they demanded freedom. The same clinic that welcomed the displaced men, women and children who came from different cities of Syria that were bombed by the regime throughout 2012 and 2013.
They ignored the love in your heart and the kindness you treated others with. They shut their eyes so as not to see all that you had offered to the country and to Syrians. Maybe they did not know that you left your quiet life in Saudi Arabia and returned to Syria at the beginning of the revolution to share the Syrians' dream of freedom and to help your country's people in their struggle for justice.
They did not know that you opposed the regime and that you fought for a political life that upheld the standards of liberty and democracy even before the revolution started. Or maybe they abducted you because they knew that, because they are the worst enemies of freedom.
Father … these criminals did not just abduct a surgeon who rejected the new oppressors. He fought in defence of the revolution and its values by saying "No" in the face of all oppressors.
No, they kidnapped the guardian of our family and the brightest of us all. They kidnapped your laughter. They kidnapped life from the face of my mother who until this day refuses to believe this tragedy. They kidnapped us all. They assassinated our days and made us prisoners of nothingness.
Your absence, father … is a crime against us, against humanity and against everyone who knew you. From friends to patients: you are the peaceful doctor whose weapon was but a mere scalpel and a smile that you used to heal the wounds of Syrians at a time where most doctors had left from the liberated areas.
You offered a warm home to anyone who sought it. A wonderful father to a family which lost the meaning of life and joy - a family that's languishing on the thresholds of an eternal wait.
A husband to a patient woman who has not lost hope since you've been gone. Every day, she writes the number of days you have been missing on Facebook. The last thing she wrote was four years and one month of your absence.
Four years have passed, with us having no means to contact you, as if you were in another world. You have been deprived of the most basic rights of a detainee - communicating with your relatives. We are forbidden from our basic rights as well - knowing your fate.
Lastly, father, allow me to apologise for the foolishness of this world, its indifference and cruelty.
And let me tell you about the latest news if you didn't know already: in the last month the world celebrated the liberation of Raqqa from Daesh by the Syrian Democratic Forces with support from the US-led coalition forces.
Don't be too happy father, because this change had no outcome on your file or your friends. Those who claim to support freedom were not interested in your case. I do not understand how they consider this liberation a victory when the fate of the first Syrians who fought against Daesh is still unknown.
Maybe I should burn this letter and use it as a beacon next to our doorbell. Maybe then you will see its light and can hold the edges of that flame to find your way to us. Maybe the distance between us then can become shorter than a dream. Maybe the distance will one day be a memory we can burn to warm us up when we meet.
