Mystery of the blood stain rvfm 200 009 p in this activity students act as lab technicians and assist investigators by examining evidence collected at a fictional crime scene students first use a presumptive blood test to determine if a substance on a stained piece of cloth may be blood and then determine the blood type of a sample collected at the scene p p students can then compare their results to those of samples provided from two suspects p rvfm forensics using simulated blood