Television and the movies notoriously overstate the tools (especially the electronic ones) at a criminal investigator’s command, to the extent juries have developed increased expectations about the availability of forensic evidence. (Here’s a fascinating study of the “CSI Effect,” suggesting prosecutors and judges need to up their game.) At the same time, many writers of crime thrillers strive to accurately portray crime scene investigations and to make their fictional detectives follow more careful procedures than often occurs in real life.
Forensic investigator Geoff Symon recently talked to crime authors about evidence. He began by dividing it into two categories:
- direct evidence, which means eye-witness accounts, with all their well-documented weaknesses and
- circumstantial evidence, which is everything else.
Symon emphasized that circumstantial evidence is still evidence, and when a tv lawyer pooh-poohs a case, saying “it’s only circumstantial,” that’s not necessarily a weakness. In truth, unless there is a reliable eye-witness, all cases are circumstantial. Fingerprints, hairs, fibers, and blood and DNA other than the victim’s are all circumstantial evidence, and the accumulation of evidence of this type, when put together in a convincing narrative, can become absolutely compelling. Circumstantial evidence can relate to a particular category of people (say, all those with blood type AB negative or having a carpet with a particular kind of fiber), or to a particular individual (fingerprints or DNA).
Says Adam Plantinga in 400 Things Cops Know says “People watch crime shows on TV so they think the police can get readable prints off just about anything—human skin, stucco walls, quesadillas,” but “only a few surfaces are conducive to the retrieval of fingerprints.” Slick surfaces, like noncoated glass, glossy paper, and aluminum are best, he says.
Two additional considerations are avoiding contamination of the crime scene and maintaining the chain of evidence. No longer do hordes of people enter the room where a body lays, tromp around in their own shoes, and depart. (In the notorious 1954 murder of Marilyn Sheppard, “Police officers, relatives, press, and neighbors [were allowed to] troop through the house.” Subsequently, this case was a basis for the movie and tv series, The Fugitive.)
Today’s investigators recognize that “whenever you leave a room you take something with you and you leave something behind,” Symon said. Thus, the importance of hair coverings, gloves, booties, and hazmat-looking suits. Cross-contamination of the crime scene was vital to the defense of O.J. Simpson. First investigators on the scene, therefore, have a particular responsibility to document it accurately with photos, video, sketches, and notes, knowing it may be contaminated subsequently.
Similarly, the chain of custody for evidence is an essential part of “preserving” the crime scene evidence. Unless a piece of evidence has been carefully tagged, and each subsequent person who handled and tested it has signed for it, criminal prosecutors cannot claim that a trace of DNA , a hair, or other physical evidence is the same bit gathered at the crime scene and not somehow introduced subsequently.
Symon and other forensic investigators help authors by describing “reality.” The challenge for the author is to subvert reality in a believable way so their story’s plot can unfold. While in real life, procedural mess-ups may mean perpetrators are never be brought to justice, this often suits the author’s fictional purposes very well.