Foundational work in social movement scholarship maintains that coming into full awareness of the scope of the injustices one faces is essential for an individual’s engagement in social movement activity. This study draws on interview data with family members who belonged to the Association of the Families of the Martyrs of the Abu Salim Prison Massacre, which began demonstrating on behalf of disappeared men in Libya in 2007. For these families of forcibly disappeared men in Libya, uncertainty and ambiguity about whether their relatives remained alive was an important catalyst for collective action in a state where political organizing was strictly prohibited by law. One of the key findings of this study is that the denial of information about a disappeared relative and the uncertainty produced by his/her absent body is an important motivation for participation in collective action. These findings suggest that the preeminent role of certainty embedded within many social movement theories about how movements develop may not extend to many types of collective action, such as movements around the disappeared in Libya. Ambiguity and uncertainty can not only be important sources of compelling social movement activity but can actually be very effective in sustaining it over time.