The Science
of Investigation
Go behind the lab door. See how hidden fingerprints are made visible and compared, how a tiny swab becomes a DNA profile, how glass and fibres are compared, why a colour-change test only says “maybe”, and why the time since a death is only ever given as a range.
- 00First Principlesp.2
- 01Fingerprintsp.4
- 02DNAp.5
- 03Trace evidencep.6
- 04Stains and testsp.7
- 05Time since deathp.8
- ✦Self-check · Make it · 5 Questions · Side Questp.9
You'll need: a notebook, a pencil, clear tape, cocoa powder, a soft brush, glitter, vegetable oil and an adult helper.
Principles
A first principle is a basic truth you can build on. The science of investigation rests on four: every contact can leave a trace, contamination is the enemy, a screening test isn’t proof and science reports what it can’t tell, too.
What do I already know?
What is necessary?
Everything in this module is built from four bricks. Rate yourself on each one, then check the example to see if your rating holds up.
What is the objective?
By the end, you’ll be able to name 3 kinds of fingerprints and how latent ones are developed, explain the ACE-V method, describe how PCR copies DNA and why contamination matters, tell class evidence from more specific evidence, explain a presumptive test, and say why time since death is a range.
- 01Name 3 kinds of prints.
- 02Explain ACE-V.
- 03Explain how PCR copies DNA.
- 04Tell class from specific.
- 05Explain a presumptive test.
- 06Explain a time range.
The list on the left is ours. Now make it yours. What do you want out of this?
How much do you know about the science of investigation right now? Shade the boxes in pencil. You'll come back at the end (p.9) and shade it again in pen.
Why start this way? When you know what you already have (1), what you need (2) and where you're going (3), you learn faster and you notice the gaps. Every module on the Criminal Investigation Path starts with these same three questions.
Fingerprints
Hidden prints are developed, then compared in four steps.
Seen, pressed or hidden
- Patent: visible, like a print in paint or ink
- Plastic: pressed into something soft, like putty or wax
- Latent: invisible, left by sweat and skin oils
The Law book’s Lens 2 explains ridge patterns and how courts treat fingerprint evidence.
Powders and fumes
Fine powder sticks to oils on smooth surfaces and is lifted with tape. In a closed chamber, cyanoacrylate (superglue) fumes form a white crust on prints. On paper, ninhydrin reacts with traces of amino acids in sweat and turns prints purple. These chemicals are for trained lab staff only.
How examiners compare
Examiners follow four steps: Analysis (is there enough detail?), Comparison (side by side), Evaluation (same source, different, or can’t tell) and Verification (a second examiner checks). “Can’t tell” is an honest, common answer.
- Where in your home would a hidden fingerprint last longest?
- Why is “can’t tell” an honest answer for an examiner?
Lift a print
With an adult, press your own finger on a clean drinking glass. Dust it very lightly with cocoa powder using a soft brush, lift it with clear tape and stick it on white paper. Which ridge pattern do you see?What does the V in ACE-V stand for?
DNA in the lab
PCR copies DNA so well that contamination is a big risk.
Four lab steps
- Collect: swab or cut a tiny sample
- Extract: free the DNA from cells
- Copy: PCR makes millions of copies of chosen spots
- Read: count the repeats at each STR spot
The Law book’s Lens 2 covers STRs and the national data bank.
Copy, copy, copy
PCR (polymerase chain reaction), invented by Kary Mullis in 1983, heats and cools a sample in cycles. Each cycle roughly doubles the DNA at the chosen spots.
| Cycles | 1 | 10 | 30 |
|---|---|---|---|
| Copies of 1 | 2 | 1,024 | over 1 billion |
Why investigators dress like that
Because PCR copies so well, a speck of the wrong DNA gets copied too. Teams wear gloves, masks and suits, change gloves between items, and keep elimination profiles from officers and lab staff so their own DNA can be ruled out. Real results usually take days to weeks, not minutes.
- Why might a tiny speck of the wrong DNA cause big trouble?
- Should real DNA results be faster, even if that adds risk?
Doubling
Start with 1 and double it 20 times (use a calculator). How many copies after 20 cycles? Why does that make a tiny bit of contamination a big problem?Starting with 1 copy, how many after 10 doubling cycles?
Trace evidence
Most trace evidence shows a possible link, not a person.
How much does it narrow things?
Class evidence fits a whole group: a common blue cotton fibre, a popular shoe tread. It can show a possible link. Evidence that narrows to one source is much rarer. Most trace evidence, like fibres, soil and glass, is class evidence.
Refractive index
Every kind of glass bends light by a certain amount, its refractive index. Labs compare tiny glass pieces by finding a liquid that bends light the same way: when they match, the glass edge seems to vanish. Fibres are compared side by side under a comparison microscope.
Overstating a link
In 1998, the inquiry into Guy Paul Morin’s wrongful conviction found that the hair and fibre evidence used against him had “little or no probative value” (Lens 5). Good scientists say clearly how common a trace is, and what it can’t show.
- What traces might you carry home from school today?
- Why should a scientist say how common a fibre is?
The vanishing rod
With an adult, lower a small heat-resistant glass item (like a borosilicate measuring rod or test tube) into a clear glass of vegetable oil. What happens to the edges? Why?A fibre comes from a very common blue cotton T-shirt. Class or specific?
Stains and tests
A screening test says “maybe”; a lab test confirms.
Two kinds of tests
A presumptive test is a quick screening test, often a colour change. A positive result means “this might be the substance; send it to the lab”. A confirmatory test in the lab is more specific. A reddish mark could be paint, rust, sauce or something else.
Why “maybe” matters
Many screening tests react with more than one substance. Some colour tests for stains can also react with certain plants or metals. That’s why investigators photograph and record first, test only a small part, and leave the rest for confirmation.
A 2009 warning
In 2009, a US National Academy of Sciences report said many pattern methods, including bloodstain pattern analysis, needed more research and clearer limits, and that some results had been overstated in court. Canadian inquiries have raised similar concerns (Lens 5).
- Have you ever been sure something was one thing, then found it wasn’t?
- Why test only part of a stain and keep the rest?
A colour-change screen
With an adult, make red-cabbage indicator (soak chopped cabbage in hot water, then strain). Add a little to vinegar, baking-soda water and lemon juice. Does one colour tell you exactly which liquid it is?A screening test is positive. What does it mean?
Time since death
Every sign changes with conditions, so experts give ranges.
Medical examiners
When a death is sudden or unexplained, a medical examiner, a doctor with special training, investigates the cause. In Alberta, this is the Office of the Chief Medical Examiner. Police investigate what happened; the medical examiner looks at the medical questions.
Many things change the clock
Experts combine several signs, but each one changes with the room temperature, clothing, body size, illness and more. So they give a range of hours, and the range gets wider the longer it has been. Other clues, like a timestamp or a witness, often help more.
No exact minute
A TV expert who names the exact minute is fiction. Real experts say what the evidence is consistent with and how uncertain it is. Lens 1 showed why cooling alone gives a range, even for a mug of tea.
- Why do you think TV shows give an exact time?
- How does saying “we can’t be sure” build trust?
What changes the rate?
Look back at your two-mug test in Lens 1, or try it now. List 4 things that could make a mug cool faster or slower. Why would that make an exact time impossible?Why do experts give a range for time since death?
Quick self-check
Six questions, covering every stop. Show your thinking. Answers are upside down at the bottom; no peeking till you're done.
1 · Name the three kinds of fingerprints.
2 · What do the four steps of ACE-V stand for?
3 · About how many copies does PCR make from 1 after 10 cycles?
4 · Why do officers give elimination DNA profiles?
5 · What does a positive presumptive test mean?
6 · Why is time since death given as a range?
Re-shade your knowledge meter
How much do you know about the science of investigation now? Flip back to p.3 and shade the meter again in pen (or shade this one). Did any of the four bricks on p.2 move from "Not yet" to "Got it"?
The glitter transfer test
How far does one touch travel?
1 · Plan
Locard said every contact leaves a trace. Test it: put a pinch of glitter on one volunteer’s hand, then have them shake hands with, pass a book to and high-five 3 other willing people. Count the glitter on each person and object.
- Volunteers agreed
- Adult helping
- Same glitter
- Clean-up planned
2 · My counts
| Contact | Glitter |
|---|---|
| Person 1 | |
| Book | |
| Person 2 | |
| Person 3 |
3 · Must-haves
- Order recorded
- Counts fair
- Second-hand traces
- Limits named
4 · What I found
Did glitter reach people who never touched the first volunteer? What does that tell you about finding someone’s DNA or fibres somewhere, and what it can’t prove?
Use craft glitter only, keep it away from eyes, and only include people who agree. Never collect anyone’s fingerprints or DNA, and keep the case made up.
5 Questions
I Still Have
This module didn't answer everything. Good. Write 5 new questions it didn't answer, one of each type below. There are no wrong questions, only ones nobody's asked yet.
★ Star the one question you'd most like answered. You'll need it on the next page.
1What I'll find out
Break your big question into 2 or 3 smaller ones you could actually answer.
2Where I'll look
3How I'll share it
Who I'll share it with:
Done by: