Since the start of the Syrian war, a remarkable 100,000 people have gone missing - a distinct figure from the estimated 400,000 confirmed killed. Of the missing, some may be alive and held secretly in prisons controlled by Bashar al-Assad’s regime. Most are probably dead, and their bodies are often buried in unmarked mass graves in territory controlled by Assad or until recently, the Islamic State. But with the Islamic State nearing defeat in Syria and Iraq, investigators are gaining access to