Problem framing is an essential yet under-explored aspect of problem-based historical inquiry. This study investigated how tenth-grade students in one AP US History class framed an ill-structured historical causal reasoning problem. Data included students' written brainstorms, students' responses to open-ended interview questions, and researcher-created causal diagrams. The analysis, grounded in both empirical data and existing literature on historiography and problem-solving, reveals three key dimensions for researching and teaching problem framing: (a) establishing the scale of the problem space, (b) identifying relevant agents and structures, and (c) establishing causal interactions. The findings underscore the importance of metacognition, particularly the need to weigh the affordances and constraints of alternative framings. The study concludes by offering relevant instructional interventions, such as explicitly teaching problem framing concepts, designing tasks to elicit problem framing, and using prompts and visualizations to help students reflect on their problem framing.