Much of forensic is deductive reasoning, but there have been more than one conviction of innocence and acquittal of a guilty man based on that. Do not call it a pure science.
Well, since it's a historical science, no theory about how a crime was committed is ever strictly proven (except when a security camera videotape shows up). But a forensic theory can become more & more secure as more facts are analyzed & as more accurate & plausible theories are developed that integrate those facts.
Also, science uses both deductive and inductive reasoning. The scientist gathers facts and steps backward (inductively) to a hypothesis about what caused them. Then she steps forward from the hypothetical cause & deductively reasons that there should be other effects out there if this hypothetical cause is in fact true. If the predictions are borne out, then we start calling it a theory and we start using it as a building block for yet more theories.
Also note that forensic scientists sometimes will conduct experiments to see whether some step in their theory of how a crime was committed is plausible, even though they're trying to reconstruct an event that happened in the past.