Crime Scene Lab teaching notes

About these notes

Crime Scene Lab is one invented case: overnight someone broke the glass of a school's trophy case, took the cup, cut a hand on the glass, and left by the side door. Four people had a key. A student processes the room the way a forensics class processes a scene, works the evidence at six Lab stations, and builds the case as claims with the evidence attached. The teacher opens the saved project and grades it.

The case has no victim. Nobody is hurt, nothing in the room is a weapon, and the blood is a dozen small drops on a floor, drawn flat, from a cut hand. The people have first names only and no real person, place, lab, agency or product is named anywhere in the app.

The lessons

Each plan has slides, a worksheet, an exit ticket with its key, a rubric and a family edition. They are linked from the Crime Scene Lab page.

The scene in one period

The room is a school hallway at night in 3D: drag to turn, scroll to move closer, and press a place to step up to it: the trophy case (the broken pane, the key in its lock, the empty shelf), the glass on the floor (four shards, one with a dark edge), the floor (a fan of drops by the case, two round drops, a trail to the door, a smear, a shoe print in the dust) and the desk (the note, a mug). It is dark until the first photo; the pointer is a flashlight in the student's hand. Every tool is a real one with a real cost: the camera at three ranges, markers, the scale, gloves, tweezers, powder, tape, the swab, and the measuring tape for the sketch, which opens the plan.

1. (0 to 10) The whole room with the camera, anywhere in the dark. Step to the case. Close-up with the scale, marker, gloves on, powder, tape, send the lift. Ten minutes on the projector covers the whole method.
2. (10 to 35) Students process the other three places on their own. Do not stop a student who picks up a shard before its photo: the log line stays, the Case chip carries it, and that is the lesson.
3. (35 to 50) The Evidence tab: every bagged item with its chain of custody in order numbers, and the log with its mistakes in red. Read three log lines aloud.

The Prints station

A lift is the suspect's actual finger seen through the tape's window, smudged where the brush went twice and fainter if it is the second lift of the same print. The student names its class (three at middle school, eight at high school), marks ridge endings, bifurcations and dots on the lift, picks a suspect and a finger from the ten-print card, marks the same features on the card and pairs them. The station counts the pairs. The call is the examiner's: identification, exclusion or inconclusive.

• The custodian's prints are on the pane, the small shard and the key, and their card is on file as an elimination set. A print on the glass that is the custodian's means nothing, and a student who excludes the custodian from the suspects has understood what elimination prints are for.
• An identification on two or three pairs is accepted. The chip it hands to the Case says the count, and the check after hand-in says whether the finger was right. Nothing in the app calls a number of points a match.
• The rules pane at the bottom writes the classifier as three rules by delta count and runs it on a whole card. The tented arch breaks it: a delta, and still an arch.

The Spatter station

A stain with a close-up and the scale opens at the bench framed close, with a 10 mm bar. The student drags a caliper across it for the width and along it for the length, to the half millimeter. At high school the angle comes from a formula the student builds from chips, so acos or the ratio upside down gives wrong numbers everywhere, visibly; at middle school the angle is picked off a chart of five shapes. The check after hand-in passes a width and length read within 0.75 mm of the stain, because half a millimeter on a stain this small moves the angle by several degrees, and it says which reading was off and works the true angle out from the stain's sizes. The strings are dragged along the drops on the floor plan; the app extends each to 150 cm and marks where they cross.

• The fan of six drops by the case came from one point, a hand shaken once, so the strings converge there and the height follows from the angles and the distances. The app calls the height an estimate and says why.
• The two round drops by the case fell straight down. The trail to the side door is a moving hand: tails toward the door, no single origin. The station says what a trail can say: where someone moved, not who.
• At high school the check passes a stain when the width and length are each within 0.75 mm and the formula is right, or when the angle is within five degrees; at middle school a picked shape within twelve degrees passes. Examiners measure the oval and leave the tail out, and the station says so; a length taken with the tail still passes, and the check names it, since it reads the angle low.

The Ink station

The note's ink and each suspect's pen go on a strip: a pencil origin line, the solvent below it, water or rubbing alcohol. The solvent climbs in real time (a chip skips the wait), the dyes separate into bands, and the student reads each band's distance and the front's off the ruler to get an Rf. Two of the four pens hold the same ink, so chromatography alone never settles the case.

Impressions, Blood and Fibers

Impressions: the shoe print by the door, measured for length and width against each suspect's soles, with a tread, a size and a wear mark. One other suspect wears the same tread in a size at least 8 mm away. Blood: a swab in paper is typed on a slide with anti-A, anti-B and anti-Rh; the wells clump by the type, and one other suspect shares it. Fibers: a thread caught on the broken edge, visible only once the pane has its close-up with the scale, under the microscope beside each jacket; one other suspect wears the same kind in another color.

Each of these is class evidence, and each station says so in its own words: a type, a tread or a fiber rules a person out and never points at one. Only the prints in this case name the culprit on their own, which is what the Stage tab's solver shows when a student takes the prints away.

The Case, and grading it

Four prompts lead: who was in the room; what happened, in what order; what the evidence does not tell you; and, at high school, what you would test next. Each answer is a claim in the student's words with evidence chips attached from the scene and the stations. A chip carries its weakness with it, no scale, 2 points, a bare hand on the item, spoiled, a mistake on the record, so the teacher sees what each claim stands on. The student says who the evidence points at and hands it in.

• Print the case from the Case tab: the plan with the sketch, every marker's two distances, the chain of custody, each station's numbers and calls, the claims with their evidence, and one line of what went wrong at the scene.
• The answer key prints from Setup only: the seed's true story, every print's source, the true angles and the origin, the pen, the blood type, the shoe and the sketch distances. It never appears in a student's copy.
• The check after hand-in, on by default, opens the true story beside the claims with every call marked right or wrong. Turn it off in Setup to run the reveal in class.
• The log is never a score. A student who undid nothing and bagged a shard before its photo did the lab; the line is there for you to read.

Setup, the seed, the Build tab and the Stage tab

Setup is one sentence with orange words: the level (middle school or high school), which of the six stations are on, whether the culprit is different for each student or fixed for the class, whether gloves stay on, whether the check shows after hand-in, and your own questions, which become claims in the Case. Hand the project out as the starter and every student's copy carries the setup.

The seed decides everything the eye can see: which suspect pushed the glass, the ridges on every card, the drops' angles, the sizes of the shoes. Two students beside each other have different answers unless you fix the culprit. The same seed always builds the same scene, so a saved case replays exactly.

The Stage tab tells the scene as a sentence with eight orange words: who, which hand, a bare or gloved hand, the way out, the pace, a note or none, a dusty or mopped floor, a jacket caught on the glass or not. The app lays the evidence to match and says whether the scene still closes and what names the culprit on its own; with gloves, a mopped floor and nothing on the glass, the ink and the blood may leave two people, and the tab says so. Apply starts the scene over, and the saved project carries the story, so a student can stage a scene and share the project for a classmate to solve.

The Build tab is the full builder, for you or for a student making a case for others. Say who did it, write what happened and the note, and put the evidence where it lies: pick a kind and press a spot in the room or on the plan for a drop (its angle and direction are yours to set), a fan of drops from a hand at a height you choose, a smear or a shoe print, or press the case, the glass or the desk for a print (whose finger) or a fiber (whose jacket). Each suspect's name, statement, pen, shoes, blood type and jacket can be changed, so two who share a pen or a tread make the case harder. The verdict at the top says whether the evidence closes on your culprit and why each line does or does not, and what to fix when it does not. Apply, save, and share the project: whoever remixes it gets your scene, dark, to solve. The answer travels in the project, as it must for the app to check the work. While a student is solving a case, the Build and Stage tabs stay shut until hand-in; a student can open them early, and the printed case then says so on its record. Lock both tabs in Setup to take that choice away.

What the app says, and does not

• Pattern frequencies: about 65 percent of prints are loops and about 30 percent whorls, the figures in the FBI's own manual. Measured populations differ (one study of 8,000 fingers found 56, 35 and 6 percent), so the app says about and the handout figure of 5 percent arches is not repeated.
• Points: there is no fixed number of matching points in the United States. The app never counts points as a score and tells a student who identifies on two pairs that examiners look for many more.
• Chromatography: a matching strip says the inks are the same kind. The app says consistent with, never wrote it.
• Blood, treads and fibers: class evidence. The app says rules out, never points at.
• Paper, not plastic: the published rule is that stains are air dried and packaged in paper. The app's consequence, that a swab in plastic spoils, is the reason the rule exists, said as a game rule.
• The height of the origin is an estimate, and the station says why: a drop's path is not a straight line, and a measurement is to the half millimeter.

Standards

No national standard covers forensics. The lesson plans tag the Common Core writing standard for the Case (W.9-10.1 and its clause b on strengths and limitations) and the geometry standard for the angle of impact (HSG-SRT.C.8, trigonometric ratios in applied problems), each read from the standards' own pages. The NGSS science and engineering practices the work is made of, analyzing data, using mathematics, arguing from evidence, are named in the plans and will be tagged when their text has been read from the source.

Texas gives forensic science its own course, TEKS §127.652 (adopted 2021, taught from 2023-24), a one-credit science course recommended for grades 11 and 12, after biology and one of chemistry, IPC or physics, with at least 40 percent of the time in investigations. The stations line up with its strands: crime scene photography, a sketch from fixed points and the chain of custody (d)(9); arches, loops and whorls, minutiae and powders (d)(10); hair and fibers under the microscope, natural against synthetic (d)(12); ink analysis (d)(14); bloodstain size, shape and distribution (d)(18); and blood typing by antigens and antibodies (d)(19).

Sources

Every fact line in the app carries a key to a row in its sources file, and a check in the app's repository fails if one is missing. The pages the lines were read from:

• The crime scene, prints and bloodstains: the National Forensic Science Technology Center's A Simplified Guide to Forensic Science (forensicsciencesimplified.org), the crime scene, fingerprint, bloodstain pattern, questioned documents and trace evidence guides.
• Fingerprint classes, deltas, frequencies and ridge characteristics: the FBI's The Science of Fingerprints, public domain, on Project Gutenberg.
• The angle of impact formula: the Simplified Guide's bloodstain principles page, and the bloodstain pattern analysis article on Wikipedia for the formula as written.
• Packaging and gloves: Crime Scene Investigator Network, Collection and Preservation of Evidence.
• Chromatography: the Royal Society of Chemistry's paper chromatography practical for ages 14 to 16, and chemguide's paper chromatography page.
• Blood typing: MedlinePlus, Blood typing; the NHS, Blood groups.
• Fibers: the fiber kinds from the Simplified Guide's trace evidence page; the three looks under the microscope from teaching pages, still to be checked against the SWGMAT microscopy guidelines.

Snags students hit

• The room is black: it is dark until the first photo. Pick the camera and press anywhere.
• I turned the room and lost the case: press The room in the row above the stage, or scroll out; the camera never leaves the hallway.
• Nothing happens when I press the glass: the camera is still in hand, or the tool is the swab. The tool row says which is picked.
• I cannot find the fiber: it shows on the pane's broken edge only after a close-up with the scale.
• The Lab station is grey: nothing has reached it. Lifts and bagged items are sent from the panel on the right; the shoe print and the drops need a close-up with the scale.
• My angle is 74 degrees for a long thin stain: the formula is built upside down, length over width, or with acos. Read the working.
• The strings do not cross: they were drawn along trail drops, which have no single origin. Draw them along the fan by the case.
• Four prints came up instead of three: the gloves were off. The fourth is the student's own and matches no card.
• I lost my marks after changing Setup: the project is rebuilt from the setup; Setup before the work, not after.

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