The murder of the young Syrian activist took place close enough to his family's home that his youngest brother heard his piercing scream. Ibrahim Abdelqader's attackers stabbed him dozens of times and left his partially decapitated corpse hanging from a doorframe.

His family and colleagues say he was killed by a secret operative from the Islamic State group who befriended him before he struck. The message from ISIS was clear: Its enemies are not safe, even across borders.

More than a month after the slaying of Abdelqader and his friend Fares Hamadi, the media collective that Abdelqader belonged to — which secretly documents life at the heart of the Islamic State group's self-proclaimed caliphate — has been forced into deep hiding.

ISIS claimed responsibility for the murders in a video message warning that "every apostate will be slaughtered silently." It was a grim riff on the media collective's name — Raqqa is Being Slaughtered Silently, a reference to the Syrian city of Raqqa that has become synonymous with IS and its efforts to build a caliphate.

Last month, the activist media group collected the 2015 Committee to Protect Journalists' International Press Freedom Award for its work in one of the most terrifying cities of the world, monitoring ISIS and countering its steady stream of propaganda with factual accounts.

Their reports from Raqqa have tackled everything from the conscription of children to the sexual slavery of Yazidi women brought from Iraq. They have documented public killings, flagged the death of Western hostages, and tracked the bombs dropped by the Syrian regime, the US, France and Russia.

Now the Oct. 29 killings of Abdelqader and Hamadi raise concerns about the safety of anti-IS activists in Turkey, a country that only recently grasped the scale of the terror threat within its own borders.

The killings extended the reach of the campaign ISIS already wages against the collective. In the mosques of Raqqa, Friday sermons regularly feature diatribes against the activists. ISIS killed three men this year that it accused of belonging to the network, although members said they actually were the father and friends of activists. On Wednesday, an activist with the group, Ahmed Mohamed al-Mousa was killed in the northwest Syrian province of Idlib by masked assailants.

"They have threatened all of us," says Abdelaziz Hamza, a co-founder of Raqqa is Being Slaughtered Silently, who traveled to the U.S. in November to receive the journalism award. "We don't know who could be the next one."

Abdelqader, 22, and Hamadi, 20, were murdered by Islamic State operatives in the southern Turkish city of Sanliurfa on a balmy night that the two friends thought would be perfect for relaxing with a water pipe.

The media activists say the man who boasts he masterminded the attack was a familiar face — Tlass al-Surur, a wide-eyed scrawny young man who had arrived from Raqqa a few months earlier. They say Surur claimed he had come to Turkey to seek work and slowly won Abdelqader's trust, enough to carry out regular visits to his home.

All Syrian media activists, whether they are documenting the daily grind of atrocities committed by President Bashar al-Assad's government or the sensational violence carried out by ISIS, take precautions.

Abdelqader was careful, with a sharp sense of digital security and a general distrust of strangers. He masked his identity by using the pseudonym Baz al-Furati when dealing with Arab-language news networks.

But IS was cunning. Surur cultivated a friendship with Abdelqader in Turkey over the course of months.

Some say Surur lured Abdelqader by introducing himself as an IS defector, which would have presented the prospect of an irresistible trove of information. But Abdelqader's brother Ahmed insists the ruse was less elaborate, saying Ibrahim simply warmed to Surur.

Ibrahim, after all, also was originally from Raqqa. He, too, was trying to make a new life for himself in Turkey, while keeping close tabs on his native Syria.

"That was enough for Tlass to get close or attempt to get close to Ibrahim," said Ahmed Abdelqader, who spent many nights in the family's home with Surur. "Ibrahim had a good heart. He cared for the men and women of his country."

Ibrahim's murder was carefully orchestrated, friends and family believe; they suspect that Hamadi, who joined the activist group Eye on Homeland one month before his death to design images and logos, happened to be at the wrong place at the wrong time.

"His only sin was that he was a friend of Ibrahim and that he would go everywhere with him," Abdelqader said. "What confirms that they had come to take revenge specifically on Ibrahim is that his body was stabbed more than 40 times."

It was about 11:15 p.m. when Abdelqader's 16-year-old brother, Amer , heard a sharp scream coming from the direction of the building across the street where his brother was settling in for a night of fun with his friends — in Surur's rented apartment.

Amer ran over, climbed three flights of stairs and knocked on the door. When there was no reply, he thought perhaps it was nothing. The bodies were not found until the next morning when Ahmed and friends came to check on why they were so late to work and found the grisly scene —his brother's body hanging from a doorway and Hamadi lying in a pool of blood, his throat slit.

"From the very first day, we knew that IS will not leave us alone, that there will be threats," said Tim Ramadan, another member of Raqqa is Being Slaughtered Silently, who uses an activist pseudonym for security reasons. "Stopping our work is an option that is definitely not up for discussion — regardless of the threats or what happens."

That same defiance is evident in Ahmed Abdelqader, 35, who broke away from the Raqqa activist group to establish Eye On Homeland.

"They thought that by killing Ibrahim they will stop our work," Abdelqader said. "I will stay here and carry on working until we go back to our country, until we go to Raqqa, until we are done with IS."

The Committee to Protect Journalists has called on Turkish authorities to bring the perpetrators to justice. "These murders show how the grave risks journalists face in Syria have metastasized across the porous border with Turkey," said Nina Ognianova, the group's Europe and Central Asia program coordinator.

Turkish police said they were looking for Surur; preliminary information suggests he crossed into Syria the morning after the crime, and activists say he has since been seen in Raqqa. Three other suspects remain at large.

Abdelqader said Surur bragged about carrying out the attack in a phone message and still sends a steady stream of threats.

CCTV footage and witness testimony cited by Abdelqader charts a rough timeline of events. It appears Hamadi was killed first, presumably by two men who entered the building shortly after Surur and Ibrahim stepped out to buy some cooking gas.

A fourth man entered the building immediately after Ibrahim and Surur returned to the apartment. Abdelqader believes the short scream marked the moment when Ibrahim discovered his friend lying dead in a pool of blood.

"I expect that Ibrahim was startled to find Fares' dead body and started screaming," Abdelqader said. "Immediately, they cut his head and started stabbing him."

The Abdelqader family since has moved, but peace of mind remains elusive.

"They've sent me several threats through WhatsApp and Facebook — threats saying that they killed Ibrahim, that they are near us, that they are watching me and it is my turn next," said Abdelqader, who carries on with his work.

"For every one of us, there is a reward of $50,000 — not as a whole group but for each member," said Mohammed Khedr, another member whose primary work is now with the group Sound and Picture.

The remaining members of Raqqa is Being Slaughtered Silently — roughly 30 people, mostly based in Syria — are taking extraordinary precautions. Those in Turkey have changed residences and avoid encounters with contacts that have not been thoroughly vetted.

Most lack passports or the documents needed to get out of the country.

CPJ says it has been working to help activists and family members relocate, but Abdelqader says more can be done.

"We are being killed. We are being threatened on a daily basis by IS," he said. "What is strange to us is that no one has showed any concern — not even the organizations that protect journalists. No state has offered us protection or offered to help protect my family. No one has given us a helping hand."