Fifth-grade students with learning disabilities (LD) and their typically achieving (TA) peers participated in an 8-week investigation about 19th-century U.S. westward migration. During their investigations, the students analyzed primary and secondary sources to understand the experiences of these emigrants and Native peoples. The analysis of source material was preceded by teacher-led discussions about the possibility of bias in evidence that affects the trustworthiness of historical documentation. Quantitative analyses showed that these investigations were associated with gains in students' knowledge about the period of westward expansion and a better understanding of historical content and historical inquiry; however, these gains were not always comparable for students with LD and their TA peers. Furthermore, misconceptions about this historical period and the processes of historical investigation were evident in students' responses before and after instruction. We discuss how design features of the instruction and its implementation may have contributed to the development and persistence of these misconceptions.