The COVID-19 pandemic forced much of schooling online and limited students' access to informal learning opportunities such as afterschool programs. The purpose of this study was to investigate how fourth- and fifth-grade students engaged in an online engineering program and what factors influenced their engagement. We drew on a four-dimensional model of student engagement to describe how students engaged in the afterschool engineering program and to identify the factors that enhanced or inhibited engagement. Using a case study design, we drew on interviews with six program mentors and ten students and observation data from weekly Zoom sessions throughout the 2020-2021 school year. We found examples of all four dimensions of engagement, but also of disengagement. The program influenced student engagement in multiple ways, including through the program mentors' social, pedagogical, and managerial roles, students' interest in and enjoyment of engineering, the drawbacks and benefits of the online learning environment and Zoom's affordances, and the nature of the program's activities. We connect these factors to the different dimensions of engagement. We conclude with a discussion of how our findings add to research on engineering education in informal online settings and offer implications for practitioners.