The main limitation of using blood groups to identify people is that they are much more effective at eliminating people and much less effective at positively identifying people. For example, about 37% of Americans are type O+.

If a person accused of a homicide has type AB blood and it matches the type found at the crime scene of a victim, the evidence for guilt is more convincing than if a match was found for type O blood. Fortunately, forensic scientists are able to identify many blood proteins and enzymes in dried blood samples.

Finding and correctly interpreting blood traces can be of utmost importance in solving the circumstances surrounding the perpetration of the crime, the actions of the victim and the perpetrator at the scene, their contribution to the perpetration of the crime, their behavior after the perpetration of the crime.

There are two different types of blood that can be collected at a crime scene: liquid and dried blood. Liquid blood evidence is generally collected from blood pools but can be collected off of clothing as well, using a gauze pad or a sterile cotton cloth.

Basic Stages for a Crime Scene Investigation — Possible Homicide

1. Approach the Scene.
2. Secure and Protect the Scene.
3. Initiate Preliminary Survey.
4. Evaluate Physical Evidence Possibilities.
5. Prepare a Narrative of the Scene.
6. Capture the Scene Photographically.
7. Prepare the Crime Scene Sketch.
8. Conduct a Detailed Search.

This can be as long as 4 to 6 months after the death, but it is usually sooner. In cases where the cause and manner of death are certified at the time of the autopsy, the autopsy, investigative, and toxicology reports can still take between 3 to 5 months to be completed, finalized, and approved by supervisors.