If you’re a family member, a friend of a victim, or following the progress of a case, the wait for a court date can be downright excruciating. That’s why it is important to understand the tedious, time-consuming process detectives face as they investigate and collect the forensic evidence needed to arrest, charge and convict the right person.

Unless a criminal is caught red-handed, there’s a complex process in pulling together a scenario about what actually happened. In addition to analyzing evidence collected at a crime scene, detectives may need to spend hours, or sometimes, days, weeks, and months interviewing witnesses and potential suspects, and requesting and waiting for judges to sign search warrants.

It’s important to understand that experienced detectives will spend hours collecting forensic evidence at a crime scene. It also can be a painstaking process to ensure that the evidence is not contaminated and, subsequently, ruled inadmissible in court.


What are the categories of forensic evidence?

In the past, most investigators have categorized those types of forensic evidence into four areas—real (or physical); demonstrative; documentary; and testimonial. With the rampant use of technology in the past couple of decades, digital evidence has pushed that number up to five.

Using an abduction and homicide as an example, here are the ways that detectives, forensic science technicians and prosecutors may categorize the evidence found at a crime scene.


Real (or physical) evidence

During a court case, you may hear an attorney ask a law enforcement official to give a rundown of the real or physical evidence in a case. Or they may present it themselves, outlining the physical evidence gathered during the investigation. Just as the name implies, this category can include what is considered concrete evidence—such as DNA linking the suspect to the case, including strands of hair, blood and other bodily fluids. This physical evidence category also can include fingerprints, footprints, tire tracks, clothing worn by the victim or suspect, weapons such as blunt objects, knives or guns, and bullet casings. 

All of these can be used as evidence in abduction and homicide cases. For instance, tire tracks and footprints can help establish that someone abducted the victim. Different types of blood (DNA) found at a crime scene can help determine if the suspect was guilty.

One of the most famous instances of the use of physical evidence at a crime scene was a bloodied glove found at the 1994 murder scene of Nicole Brown and Ronald Goldman. Prosecutors considered the glove critical physical evidence in establishing O.J. Simpson’s guilt. However, the use of the glove in the case backfired when it appeared that it didn’t fit Simpson. 

Some law enforcement departments will make a distinction between different types of physical evidence. For instance, they may consider some physical evidence as non-organic; shoe prints and fingerprints, fibers, glass shards, firearms, and accelerants may fall into this sub-category. Those types of evidence would be considered different from biological evidence, such as DNA evidence (organic), including blood, saliva, and hair.


Demonstrative evidence

This category of evidence sets the scene of the crime. If the suspect claims it would have been physically impossible to travel from his job and physically appear in the area where witnesses said he grabbed the victim, demonstrative evidence might include a map from his place of employment to the area, along with miles, in order to show the jury that yes, it was absolutely possible. 

Demonstrative evidence might also include a map, diagram, or model of the area where the suspect is accused of dragging the victim out of his car, to dispute the defense attorney’s argument that the area is too heavily wooded for eyewitnesses to claim they saw the suspect. A display of the crime scene might also be presented for the jury. 

Crime scene investigators will take photographs, videotape, and sketch the area to support demonstrative evidence presented in a case.


Documentary evidence

Let’s say the homicide victim’s engagement was announced that morning in the local newspaper. Less than eight hours later she was found dead. During a search of her home, detectives discovered her personal writings. Including a journal. She made several journal entries, describing how the suspect repeatedly called her home and workplace, how she had seen him parked outside her home. This documentary evidence establishes her growing fear for her safety as well as the suspect’s obsessive, irrational state of mind. This type of evidence also would be presented in a court case.


Testimonial evidence

This type of evidence provides the judge and jury with anything pertaining to the case which was seen or heard before, during or after the criminal act. So, if two people witnessed the suspect physically forcing the victim into his car, they will be called to testify. Anyone who said they heard screaming, shouting, gunshots, etc., also will be asked to testify in most cases.


Detectives not only will seek out testimony from witnesses in the area, they also will interview friends, relatives, neighbors and co-workers of the victim and possibly suspects to gather testimonial evidence. Again, this can be a time-consuming process to ensure that all avenues of information are exhausted.


Digital evidence

Digital evidence only recently has emerged as one of the core categories of evidence. Several decades ago, this type of evidence would have been considered inadmissible in court. However, with the widespread use of smartphones and other digital devices, GPS, and social media, police are increasingly relying on text messages, geolocation, phone calls, emails, and other digital communications to help support a case.

A judge famously allowed the use of GPS evidence in the 2004 murder trial of Scott Peterson to help establish his movements following the disappearance of his wife. Peterson was later found guilty in the murders of his wife and unborn son.

In the case of an abduction, investigators could use cell phone towers to help determine the location of the victim and, quite possibly, locate them before the criminal has time to rape, assault or murder them. 


Seeking justice through forensic evidence

As with the admissibility of digital evidence, law enforcement officials are continuing to hone the process of collecting, preserving, and analyzing evidence to ensure that victims of crime can have a shot at justice.