On TV police dramas and movies, viewers get to watch as investigators find and collect evidence at the scene of a crime. Crime show characters can make blood appear as if by magic and piece together seemingly unrelated clues to solve crime. Many of us believe we have a pretty good grip on the process. But does Hollywood get it right? Do crime scene investigators follow their DNA samples into the lab? Do they interview suspects and catch the bad guys, or is their job all about collecting physical evidence? What really goes on when a CSI "processes a crime scene"? Let’s find out.

A typical crime scene

Crime scene investigation is the meeting point of science, logic and law. "Processing a crime scene" is a long, tedious process. It involves careful consideration and documentation of the conditions at the crime scene. It also involves collecting any physical evidence that could possibly explain what happened and point to who did it. There is no typical crime scene. There is no typical body of evidence or set of clues. And there is no typical investigative approach.

At any given crime scene, a CSI might collect dried blood from a windowpane -- without letting her arm brush the glass in case there are any latent fingerprints there. Or she might lift hair off a victim's jacket using tweezers so she doesn't disturb the fabric enough to shake off some odd-colored dust in the folds of the sleeve. All the while, the physical evidence itself is only part of the equation.

Image courtesy © Brand X Pictures/William Fritsch Collecting evidence at a crime scene

The ultimate goal is the conviction of the person behind the crime. So while the CSI is poking, prodding, scraping and scouring through stuff without disturbing any trace evidence, she's also considering all of the necessary steps to preserve that evidence. She’s also thinking about what the lab can do with this evidence in order to reconstruct the crime or identify the criminal. And the entire time she’s doing her job, she has to keep in mind any legal issues involved in making sure this evidence is admissible in court.

Who's at the Scene? Many people performing a variety of jobs will be on hand at a crime scene, including the following: Police officers are typically the first to arrive at a crime scene. They arrest the perpetrator is he's still there and call for an ambulance if necessary. They are responsible for securing the scene so no evidence is destroyed. The CSI unit documents the crime scene in detail and collects any physical evidence. The district attorney is often present to help determine if the investigators require any search warrants to proceed and obtain those warrants from a judge. Specialists (entomologists, forensic scientists and forensic psychologists) may be called in if the evidence requires expert analysis. Detectives interview witnesses and consult with the CSI unit. They investigate the crime by following leads provided by witnesses and physical evidence.

The investigation of a crime scene begins when the CSI unit receives a call from the police officers or detectives on the scene. The overall system works something like this:

The CSI arrives on the scene and makes sure it is secure. Securing the crime scene -- and any other areas that might later turn out to be part of the crime scene -- is crucial. A CSI really only gets one chance to perform a thorough, untainted search -- furniture will be moved, rain will wash away evidence, detectives will touch things in subsequent searches, and evidence will be corrupted. Securing the scene involves creating a physical barrier using crime scene tape or other obstacles like police officers, police cars or sawhorses. It’s also important to remove all unnecessary personnel from the scene. A CSI might establish a "safe area" just beyond the crime scene where investigators can rest and discuss issues without worrying about destroying evidence.

Details While searching the scene, a CSI is looking for details including: Are the doors and windows locked or unlocked? Open or shut? Are there signs of forced entry, such as tool marks or broken locks? Is the house in good order? If not, does it look like there was a struggle or is the homeowner just messy? Is there mail lying around? Has it been opened? Is the kitchen in good order? Is there any partially eaten food? Is the table set? If so, for how many people? Are there signs of a party, such as empty glasses or bottles or full ashtrays? If there are full ashtrays, what brands of cigarettes are present? Are there any lipstick or teeth marks on the butts? Is there anything that seems out of place? Maybe there's a glass with lipstick marks in a man's apartment, or the toilet seat's up in a woman's apartment. Is there a couch blocking a doorway? Is there trash in the trashcans? Is there anything out of the ordinary in the trash? Is the trash in the right chronological order according to dates on mail and other papers? If not, someone might have been looking for something in the trash.

Once the crime scene is secured, the next step is to get the district attorney involved. This is a must -- if anyone could possibly have an expectation of privacy in any portion of the crime scene, the CSI needs search warrants. The evidence a CSI recovers is of little value if it's not admissible in court. A good CSI errs on the side of caution and seldom searches a scene without a warrant.

She does an initial walk-through to get an overall feel for the crime scene, finds out if anyone moved anything before she arrived, and generates initial theories based on visual examination. She makes note of potential evidence. At this point, she touches nothing – she is only using her eyes, ears, nose, some paper and a pen. Crime scenes are three-dimensional. CSIs should remember to look up. Also, if a CSI shines a flashlight on the ground at various angles, even when there's plenty of lighting, she'll create new shadows that could reveal evidence.

Next, the CSI thoroughly documents the scene. The goal is to create a visual record that will allow the forensics lab and the prosecuting attorney to recreate an accurate view of the scene. The CSI uses digital and film cameras, different types of film, various lenses, flashes, filters, a tripod, a sketchpad, graph paper, pens and pencils, measuring tape, rulers and a notepad at this stage of the investigation. In crime scene investigation, opinions don't matter and assumptions are harmful. When describing a crime scene, a CSI makes factual observations without drawing any conclusions. She documents the scene as a whole and documents anything she has identified as evidence. She still touches nothing.