Using information provided by forensic crime scene detective Guy Olivieri, my partner 'in crime' at the NJ SCBWI annual conference in June, we're going to dispel the erroneous information that Hollywood has led you to believe is how the real world works and that shows up in (too) many novels.
As soon as an officer asks you a question, you do NOT get a Miranda warning. Take A.I.M. for the truth- Arrest > Interrogation > Miranda. Officers are allowed to question you at the scene, in the car, where ever because you are not under arrest. Clues and valuable information are gathered just by 'chatting' and police are not required to Mirandize you until you are officially under arrest and are being interrogated.
Another thing Hollywood gets wrong, says Guy, is that forensic evidence is not always available. Crime savvy criminals know to wear gloves a lot of the time. Add to that it takes sometimes months for evidence to be analyzed and run through various databases for 'hits' (where it matches up with other crimes or people). And if there is no DNA or fingerprints on file, there can be no comparison. (My mom wouldn't be in any database, but I would because I was fingerprinted when I applied for a teaching job. Guess I'll have to get her to do the foul deed....).
The way matches are made, he explained, is prints are taken and uploaded into the State Automated Fingerprint Identification System (AFIS) or the Integrated Automated Fingerprint Identification System (IAFIS) which is connected to the FBI. This system has access to 70 million criminal fingerprints, and 34 million civil fingerprints (if you're a teacher, government worker, etc. and had your prints taken for security purposes, you're in. With me.). It takes around 20 minutes to compare a card with 10 fingerprints to others in the system. For a single print, it takes a bit longer, around 45 minutes, if the fingerprint quality is good. Sounds good, right? Not all police departments have access to the system. Between lack of access to technology and understaffing for the time-consuming and tedious evidence collection and analysis, most crimes aren't solved more quickly, and some not at all.
One thing that drives Guy crazy is when a show has detectives getting fingerprints off clothing or other rough surfaces. Not happening, unless your novel is a sci fi and they've developed technology in the future that can do this.
Like fingerprints, DNA is collected. There are 15 million samples-and there are 7.5 billion people. Odds are against a match unless your perp is already in the system. However, all prison inmates must surrender to DNA and fingerprint sampling, and now some states require both DNA and fingerprint samples be taken especially from suspects in violent crimes.
RIP chalk outline guy. Unless you're writing a 1920s murder mystery, he's long dead and gone.
Blood spatter: There are various types. Transfer is from the source (victim) which is moved by a handprint, foot print, etc. Passive is when it drips or runs by itself. High velocity looks like it was sprayed with a mister, medium velocity has larger drops and splatter, and arterial spurt looks like someone spurted ketchup against the wall. (Unscientific, I know, but it helps give you an idea).
If you want to determine the direction of where a blood spot originated, the formula is: Angle = SIN-1 (Width/Length). Yeah, don't ask me, it's been decades since I did that kind of math. If the spot is elongated (looks like it has a tail, kind of like a comet), the tail points in the direction of travel.
Finally, here are some terms relating to the stages of death (more than just rigor. Who knew death was so complicated?).
Pallor mortis - a paleness that presents in light-skinned or white people within 15-25 minutes after death because there is no circulation.
Algor mortis - lowering of body temperature after death.
Rigor mortis - After 3 hours, the body begins to stiffen, reaching the maximum stiffness after 12 hours. This disappears about 3 days later.
Livor mortis - when all the blood settles in the lowest part of the body, creating a purplish-red discoloration. Even if the body is moved, the discoloration remains.
Decomposition, the breaking down of the body, occurs next, followed by skeletonization.
Next week, the final part: mistakes for writers to avoid.
*Above information, except for my notations, were obtained from Det. Olivieri's extensive knowledge and his handout, Scene of the Crime: How Experts Handle A Crime Scene. He lists The Manual of Detection by Jedediah Berry and Det. Sgt. Joseph L. Giacalone, ret., http://joegiacalone.net/cold-case-squad/ as his sources.