# The Casing Match and DNA This is the forensic spine of the ballistics case, and the reason the "inconclusive bullet" talking point does not do what its promoters claim. ## Two different objects Firearms evidence here comes in two forms, and conflating them is the central error online: 1. **The fired cartridge casing** — the brass case left after firing, recovered **with the rifle along the escape route** (a bolt-action doesn't auto-eject onto the roof — see [[No Casing or Rifle on the Roof]]). This was **positively matched** to [[The Rifle]] by firearms examiners, and carried **Robinson's DNA**. 2. **The bullet-jacket fragment** — a piece of the projectile recovered from the scene. The ATF found it **inconclusive**: could not be identified *or excluded* as fired from the rifle. Four lead fragments were unsuitable for comparison. See [[The Inconclusive Bullet]]. A **positive casing match** and an **inconclusive fragment** are entirely consistent — fragmenting rifle rounds routinely defeat bullet comparison (inconclusive results occur in roughly 20% of such cases), while the casing, stamped by the firing mechanism, still matches. ## The DNA Two DNA records tie Robinson to the physical evidence: - **From the probable-cause affidavit:** Robinson's DNA on the **trigger**, the **casing**, and the **towel** the rifle was wrapped in. - **From Day 2's Exhibit 31** (the FBI Laboratory report): the **towel (item 7.1)** and the **screwdriver from the perch (item 8.1)** were each read as a **two-person mixture** with Robinson as the **major** contributor — likelihood ratios of **1.7 octillion** (towel, [05:11:02](https://www.youtube.com/watch?v=y6ofpz6ReuE&t=18662s)) and **30 quintillion** (screwdriver, [05:11:41](https://www.youtube.com/watch?v=y6ofpz6ReuE&t=18701s)), the FBI's "**very strong support for inclusion**." The minor contributor on both is Robinson's roommate [[Lance Twiggs]] (~5% and ~11%). **Full results table + reliability caveats: [[The DNA Didn't Match]].** > 🎥 **Forbes Breaking News:** the FBI investigator walks through the DNA evidence recovered in the case — Robinson, the roommate, and the mixtures — in open court.  Combined with a **shoe-impression match** and his vehicle on campus, this is direct physical linkage — not the "no forensics tie him to it" picture the Day-1 cross-examination alone might suggest (Day 1 predated the forensic testimony). See [[2026-07-06 - Preliminary Hearing Day 1|Day 1 Legal Review]] and [[They Have the Wrong Person]]. ## Day 2: the DNA examiner, cross-examined The FBI DNA that ties Robinson to the trigger, casing, and towel was tested at the FBI lab; on **Day 2** of the preliminary hearing the **defense** — not the State — called the FBI examiner, **Amanda Bakker**, and pressed the method's limits, producing a two-way record: - **What the defense drew out.** STR typing counts the **length** of the repeat regions — Bakker's own **"boxcar" analogy**, i.e. counting the boxcars on a train — **not the underlying sequence** (Day 2 [05:37:00](https://www.youtube.com/watch?v=y6ofpz6ReuE&t=20220s)); and the **PCAST report** (President's Council of Advisors on Science and Technology) questioned whether reliability had been established for parts of forensic practice ([05:51:25](https://www.youtube.com/watch?v=y6ofpz6ReuE&t=21085s)). - **What the State drew back.** Bakker countered that the **PCAST report was not written by forensic-DNA experts** ([05:55:02](https://www.youtube.com/watch?v=y6ofpz6ReuE&t=21302s)) and defended her lab's **accredited, validated** protocols. Net: the DNA linkage is real and lab-validated, but the defense preserved a **reliability-and-limits** argument for trial. See [[Insufficiency of the Evidence]] and [[Chain of Custody Problems]]. ## Day 4: the ATF firearms examiner confirms the casing match — and its limits On **Day 4** the **defense** called the ATF firearms/tool-mark examiner, [[Samantha Karner]], who did the comparisons "at the request of the state of Utah" and authored the report (Exhibit 32) [06:24:59](https://www.youtube.com/watch?v=89ZKQIHaJpM&t=23099s). Her testimony sharpened the **two different objects** distinction above: - **The cartridge case is a positive identification** — Karner "identifies the cartridge case as being fired in the firearm" [06:31:27](https://www.youtube.com/watch?v=89ZKQIHaJpM&t=23487s). The casing match is now confirmed by the examiner who made it. - **The bullet-jacket fragment (Item 6A) stays inconclusive** — "could not be identified or excluded" [07:11:04](https://www.youtube.com/watch?v=89ZKQIHaJpM&t=25864s). Same witness, opposite result — because the objects are different, not because the case is weak. See [[The Inconclusive Bullet]]. The defense used her to preserve a **subjectivity-and-error-rate** attack on the discipline itself: firearms/tool-mark identification "is subjective in nature" [07:04:29](https://www.youtube.com/watch?v=89ZKQIHaJpM&t=25469s); she cannot claim uniqueness, individualization, or a zero error rate [06:30:41](https://www.youtube.com/watch?v=89ZKQIHaJpM&t=23441s)–[06:33:32](https://www.youtube.com/watch?v=89ZKQIHaJpM&t=23612s); and her own **2023 bullet proficiency test carried a 20.4% error rate** [08:03:10](https://www.youtube.com/watch?v=89ZKQIHaJpM&t=28990s), against the backdrop of the **NAS (2009)** and **PCAST (2016)** critiques [07:57:23](https://www.youtube.com/watch?v=89ZKQIHaJpM&t=28643s). On State cross (McBride), she countered with an **ANAB-accredited**, proficiency-tested lab whose conclusions were **verified by a second analyst** [07:51:53](https://www.youtube.com/watch?v=89ZKQIHaJpM&t=28313s) and a method "routinely admitted" in court [08:04:48](https://www.youtube.com/watch?v=89ZKQIHaJpM&t=29088s). Net: as with the DNA, the **casing linkage is real and examiner-confirmed**, while the defense keeps a subjectivity/error-rate argument alive for trial. See [[2026-07-09 - Preliminary Hearing Day 4|Day 4 Legal Review]] and [[2026-07-09 - Preliminary Hearing Day 4 Transcript|Day 4 Transcript]]. ## Day 5: the ATF DNA examiner — presence, not source On **Day 5** the defense called the ATF's **DNA Section Chief, [[Caitlin Oliver]]** — the DNA analog to Karner — to establish the **limits** of the DNA linkage: her lab reports **likelihood ratios, not source attribution**; DNA "**can't speak to the activity that led to the deposition**" or its timing [02:03:02](https://www.youtube.com/watch?v=r3OUG7n37po&t=7382s); and "**a person's DNA may be present when they were not involved in the crime**" [02:12:52](https://www.youtube.com/watch?v=r3OUG7n37po&t=7972s). But on **McBride's cross**, as with the casing, she **confirmed the inculpatory conclusion** — the profiles are "**at least 1,000,000,000,000 times more likely … [if they] originated from Tyler Robinson as the major contributor**" [02:46:28](https://www.youtube.com/watch?v=r3OUG7n37po&t=9988s), the lab's reporting **cap**. **Net:** the **DNA→Robinson linkage holds** for probable cause; the defense keeps a **presence-not-action / Rule 702** argument for trial. See [[The DNA Didn't Match]], [[Caitlin Oliver]], [[2026-07-10 - Preliminary Hearing Day 5|Day 5 Legal Review]]. > **In plain terms:** > - **STRmix** is the **software** the FBI lab uses to make sense of a DNA **mixture** (DNA from more than one person on the same item). Rather than a person eyeballing whose DNA is whose, the program models the mixture mathematically and produces the likelihood-ratio statistic. Because it's a probabilistic model, it **doesn't give the exact same number twice** (it can vary within roughly a factor of 10). > - **PCAST** is the **President's Council of Advisors on Science and Technology** — a White House science panel whose **2016 report** questioned whether some forensic techniques, including **complex, low-level DNA mixtures** like these, had been scientifically validated well enough to be relied on in court. ## Day-4 exhibit (July 9) ![[dna analysis known sample from tyler robinson exhibit 30.png|560]] *The **ATF forensic-biology (DNA) report, Plaintiff's Exhibit 30** — listing the Mauser rifle, the .30-06 casing and cartridges, and the **known DNA reference sample from Tyler Robinson** used for comparison.* ![[state exhibit dna evidence level of support robinson and twiggs.png|560]] *The **FBI DNA report (Exhibit 31)**: the **towel from the rifle** (Item 7) returned a likelihood ratio of **1.7 octillion** — "Very Strong Support for Inclusion" of T. Robinson (assumed contributor: Twiggs); the **perch screwdriver** (Item 8) returned **30 quintillion**. A. Robinson and M. Robinson were **excluded**.* ![[state exhibit 31 dna evidence items reviewed.png|560]] *The evidence items reviewed for DNA (Exhibit 31).* ![[dan evidence lab report likelihood ration tyler robinson.png|560]] *The likelihood-ratio findings for Tyler Robinson, as displayed in court.* --- **Answers / relates to:** [[The Inconclusive Bullet]] · [[They Have the Wrong Person]] · [[Robinson Is a Patsy]] · [[The Confession Is Coerced or Fabricated]] **Sources:** [[2025-09_charging_police-affidavit-full.pdf|Police affidavit]] · [[defense_motion-to-vacate-or-continue-prelim.pdf|Defense motion]] · [[state_opposition-to-motion-to-show-cause.pdf|State's opposition]] · [[Sources]]