Crime Scene for Mathematics Investigation (CSMI) was created as a small group, cooperative, self-selective grouping strategy to allow students to explore mathematics based on their own understanding and perceived readiness, much like the concept and application of literature circles in language arts. In essence, these self-chosen small groups met as mini-learning communities to support one another in developing mathematical reasoning strategies through open discussion while problem solving. Three tiered sections of CSMI dealt with similar mathematical content or themes; these sections were called Yellow Tape, Detectives, and Forensics. The author described the sections to the students from the perspective of police officers who have different duties but must still work together. Just as there are different types of police officers who work to solve crimes, different groups of mathematicians in the class must work together to learn math based on a common theme. Although CSMI might look different from classroom to classroom, four basic procedures are involved in implementing CSMI: (1) Identify Learning Goals; (2) Purposefully Select Problems; (3) Elicit Evidence of Learning; and (4) Adapt Instruction. Creating a classroom structure that allows the teacher to promote students' mathematical reasoning, formatively assess learning, and adapt instruction to support all students is critical, regardless of students' current understandings or readiness. CSMI created a structure within the author's classroom by which he could accurately gauge student learning and address misconceptions in a timely manner while helping students develop relational understanding. Furthermore, students believed that they could be successful in mathematics. (Contains 5 figures.)