The Web allows learners to investigate any question with Web resources to construct wider and deeper knowledge. In such an investigative learning process, it is important for them to elaborate their question, which involves decomposing it into the sub-questions to be further investigated. Such question decomposition corresponds to creating a learning scenario that implies the questions to be investigated and their sequence. However, it tends to be insufficient. This paper addresses the issue of how to promote the question decomposition. Our approach is to model the Web-based investigative learning process. In this model, the learning scenario is represented as a tree of questions to be investigated. We have also developed an interactive learning scenario builder (iLSB), which provides some scaffolds for building the question tree as modeled. This paper also reports results of our case study suggesting that iLSB has a potential to make an elaborate investigative learning process.