Researching criminal justice topics can be quite different from researching the law. There is overlap, of course, but sometimes not very much at all depending on the specific question.
The field of criminal justice also has many of its own research resources that are completely separate from legal research resources.
Both fields of study have roots and branches in law, courts, public policy, government, history, sociology, and psychology, but they are different fields of study. One could say that criminal justice is a subset of “the law,” but it’s also its own field, and it is, of course, as interdisciplinary as any field of study is nowadays.
The Portland State University library has this Research Guide for Legal Resources, but it confuses the subjects, too. This is actually a “Research Guide for Criminal Justice Research.”
Keep in mind, also, that the term “primary source” refers to one set of documents when performing basic legal research and it refers to a completely different set of documents when historians, criminologists, and when some legal scholars speak of “primary source.”
My advice: ask a librarian when you begin developing your research strategy. It wil save time and aggravation.