With the pervasiveness of mobile technologies, witnesses have the opportunity to mediate up-close and seemingly truthful recordings of events. As such, "witness videos" have become prominent in news reports and serve as authoritative resources in the construction of memory. However, once they are uploaded to video-sharing sites and popular archives such as YouTube, they are being reassembled and remixed by distinct actors, along the lines of their own ideological agendas. Focusing on the chemical attack on Ghouta, Syria, this article investigates how witness videos are represented by uploaders (ranging from established media to activists) and structured by the affordances and sociotechnical practices associated with the platform. Hence, we argue, although the future memory of the attack is constituted by witness videos, it is powerfully shaped by various actors, both human and nonhuman. These mechanisms of memory construction are empirically explored by qualitative and quantitative analyses of meta-data and (remixed) content.