ISTANBUL — The killing of the young Syrian activist took place close enough to his home in southern Turkey 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 the attack. The message from IS 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.

IS claimed responsibility for the killings 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.