This study investigated the development of source use through a series of writing assignments for a university liberal arts course. Participants were two undergraduate students, whose L1 were Japanese and Chinese. Citation analysis revealed that the final versions of term papers used references with higher reliability and various citation signals compared to the previous assignments. Nevertheless, the final essays included over-citations, where source attributions were unnecessary, and under-citations, where distinctions between original texts and the students' own ideas or expressions were difficult. These results suggest that source-based writing is considerably challenging for novice writers and requires persistent, long-term support.