How a Criminal Defense Lawyer Challenges Forensic Evidence

Forensic evidence arrives in court with the aura of a lab coat. White walls, calibrated machines, technical jargon that sounds like a foreign language spoken through a microscope. Juries lean in. Judges take notes. Prosecutors smile like they brought the sun. And yet, on the defense side of the aisle, you learn this simple truth: every test is a story about how it was done, by whom, under what conditions, and with what assumptions. Stories can be cross-examined.

A good criminal defense lawyer does not fight science. That is a fool’s errand. We fight the leap from data to certainty, from messy reality to tidy labels like “match” or “positive.” We fight human error, cognitive bias, sloppy chains of custody, unvalidated methods, gilded résumés, and the temptation to call a probability a proclamation. The work is technical, unglamorous, and often tedious. It also decides liberty.

The starting line: case theory meets lab theory

By the time the prosecutor sends over discovery, the lab reports have usually already locked in their phrasing. “Consistent with.” “Cannot exclude.” “Indistinguishable.” Sometimes the conclusion is stronger. “Identified.” If you are new to this: none of those words have fixed, universal meanings across disciplines. A firearms examiner’s “identification” is not the same species of confidence as a DNA analyst’s “inclusion.” Language in lab reports sometimes grows out of tradition rather than mathematics. That is the first pressure point.

My first pass through a forensic report is simple. I circle every conclusion, then ask what raw materials make it possible. If the lab won’t give me the underlying data, I file motions. If they give me data in an unusable format, I ask for it again. You cannot cross-examine adjectives. You can cross-examine chromatograms, stutter peaks, electropherograms, batch controls, calibration logs, temperature logs, reagent lot numbers, proficiency testing results, and the analyst’s bench notes. Those materials tell you how sausage became scripture.

Chain of custody: the quiet backbone

Romance novels have plot twists. Forensic evidence has handoffs. Every transfer from scene to bag, bag to locker, locker to courier, and courier to bench is a chance for contamination or mix-up. Juries assume a sample rides a conveyor belt; in real life, it rides on human habits. A broken seal, a mislabeled tube, a bag stored near a wet item, or a swab dried on a contaminated surface will not announce itself. You have to go looking.

The question is rarely “Did someone forge evidence?” It is almost always “Could and did ordinary mistakes change the sample?” I once had a case where a mud-caked sneaker traveled unbagged in the trunk of a patrol car packed with items from two scenes. The officer was not malicious, just hurried. The lab later reported trace soil consistent with the second scene on the shoe. Consistent with is doing heavy lifting there.

Defense practice is detail work. You compare the times on evidence seals to the intake logs. You look at whether exhibits sat unrefrigerated, whether the lab dried items in a shared hood, whether the same technician handled known samples and case items back to back without changing gloves. You do not have to prove a contamination happened. In many jurisdictions, it is enough to show a reasonable possibility the protocols created the opportunity. Reasonable doubt lives there.

DNA: powerful, and more complicated than TV suggests

DNA is often treated as the gold standard. When you dig into mixed profiles, low-template samples, touch DNA, and probabilistic genotyping software, the gold gets streaked with fingerprints.

Mixtures are the heartburn. If a swab pulls DNA from multiple contributors, the analyst has to decide how many contributors are present, which peaks are true alleles, which are stutter artifacts, and whether to use assumptions about relatedness. Then come likelihood ratios that can stretch into astronomical numbers. Those numbers can be very persuasive, but they depend on modeling choices that are neither obvious nor neutral.

Defense work on DNA has a rhythm:

Get the raw electropherograms and lab’s interpretation worksheets, not just the summary page. Examine the stochastic threshold and how the lab treated peak drop-out and drop-in. Low template DNA is a minefield for allelic drop-out. Review the lab’s validation studies for mixtures and for the specific kit used. Shortcuts here matter. If software was used, request the version history, user manual, and audit trail. Parameters and user interventions can sway outcomes.

I have cross-examined analysts who conceded that a different, equally defensible number-of-contributors assumption would flip an inclusion to “cannot determine.” Juries respect humility in science. They also appreciate that DNA on an item does not tell you when it arrived, how it got there, or whether it reflects primary transfer, secondary transfer, or a sneeze on a crowded bus. In a burglary case, a client’s DNA on a windowsill says something. It does not, by itself, say he climbed through last night.

Latent fingerprints: uniqueness meets subjectivity

Fingerprint examiners use a methodology called ACE-V: analysis, comparison, evaluation, and verification. The method sounds rigorous. The devil sits in the thresholds. There is no universal numerical standard for how many matching features equal an identification in the United States. Examiners talk about ridge detail, minutiae, and sufficiency, then arrive at a conclusion that often reads as categorical.

When I challenge latent prints, I ask for the images, not just the charts. I want to see the quality, the smudging, the distortion, whether the latent came from a curved surface, whether the examiner used enhancement software, and how they documented discrepancies. Any honest examiner will admit that distortion changes what features appear, that different examiners may disagree about which features are present, and that confirmation bias is a risk if they knew the suspected donor’s name while analyzing.

The most telling moments come when an examiner’s blind verification turns out to have been not very blind. Some labs use “technical review” procedures where the second examiner knows the first examiner’s conclusion. That is not a blind test. It is a box checked on a form.

Firearms and toolmarks: patterns and probabilities

Saying a bullet “matches” a firearm suggests a precision that the underlying science does not fully support. Toolmark analysis compares striations and impressions, looking for “sufficient agreement.” That phrase is familiar because it appears in many lab SOPs. It is also subjective. Two different examiners can legitimately disagree about sufficiency, especially on marginal samples.

Modern labs have been adding 3D imaging and attempting to quantify similarity, which is a step in the right direction. Still, many casework opinions rest on side-by-side microscope comparisons and training-driven judgment. A defense challenge usually focuses on the clarity of the marks, subclass characteristics that can mimic individuality, the number and quality of test fires, and whether the examiner documented dissimilarities or just similarities. A finding that glosses over differences is ripe for cross.

I have asked examiners to physically show a jury the regions they considered “high-quality” and to explain how they excluded the possibility of subclass carryover from tool manufacturing. Jurors do not need a PhD to see when the picture looks fuzzy and the confidence looks polished.

Drug chemistry: presumptive tests, confirmatory tests, and shortcuts

Street-level drug cases often start with color tests. Those are presumptive, not confirmatory. They can react to a family of substances and have false positives. The real work happens with gas chromatography mass spectrometry, FTIR, or another confirmatory method. When https://www.dreishpoon.com/criminal-defense/ the state leans on a color test or a rushed FTIR reading without standard libraries, you have openings.

Lab backlogs produce a related problem: rushed analysts, batch processing, and sometimes the use of “representative sampling.” In weight-driven felonies, analysts might test a fraction of seized items but report the total weight as a single controlled substance. That can be defensible if the protocol is sound and homogeneous packaging is proven. It is less defensible when items vary visibly or were commingled after seizure. The defense asks for the sampling plan. A prosecutor’s shrug is not a plan.

Calibration and quality control are also rich soil. A GC-MS with a drifting calibration produces garbage. You would be surprised how often calibration logs show gaps or failed checks. A criminal defense lawyer who can read those logs becomes dangerous.

Digital forensics: the illusion of completeness

Phones and computers look authoritative when imaged and charted. Timelines, geolocation points, message threads, file hashes, all wearing a badge of precision. The pitfalls are different from wet labs but just as common: acquisition method, parsing choices, timezone handling, tool limitations, and data that lives outside the easy pathways of forensic suites.

One case turned on a timestamp. The state’s analyst testified that a video clip placed my client at a location at 8:42 p.m., based on device metadata. The device had auto-updated its timezone during travel, and the extraction tool applied a daylight saving offset twice in a specific view. When we dug into the raw timestamps, the video was an hour off. That moved it outside the window of the alleged offense. The analyst had not lied. The tool had simplified.

Defense work in digital evidence involves an uncomfortable amount of reading user manuals and experimenting with test datasets. The goal is not to confuse, but to show that the data tells multiple possible stories. It is even better if you can demonstrate a known artifact that aligns with your theory.

Bloodstain pattern analysis: velocity words, wobbly foundations

Nothing in forensics has taught me more humility than bloodstain pattern analysis. It looks mathematical. Angles, arcs, origin points, words like “low velocity spatter.” In practice, much of it is impressionistic. Experienced analysts can do good work on large, clear scenes. Smaller scenes, mixed mechanisms, or lack of context breed speculation.

I once watched an analyst call “high velocity” spatter on a shirt from photographs. The lab never tested for gunshot residue, and the case had no firearm. On cross, the analyst admitted that high velocity is a label often used for gunshots, but also seen with machinery or ruptured blood-bearing organs, and that photographs compress depth. You could feel the jury’s trust deflate.

The best defense tactic here is to force specificity. Which stains, measured how, with what assumptions, and what alternative mechanisms were considered? If the answer is vague, you have made progress.

The human factor: bias, blinding, and cognitive pitfalls

Across disciplines, the most pervasive threat is not malice. It is human nature. Analysts are people. They prefer being helpful to the case agents with whom they work daily. They like to be correct. If they receive a note saying “suspect confessed, check these items,” they are more likely to see what aligns. That is not a moral failing. It is psychology.

Defense lawyers push for context management and blinding. A lab should wall off irrelevant case details from the analyst. Verification should be truly independent. Proficiency testing should include blind tests disguised as casework, not announced exams scheduled weeks ahead. When a lab resists, that resistance itself is telling. Juries understand why you do not tell a taste tester which cup is Pepsi.

Discovery battles: getting what you need to do the job

Many forensic weaknesses stay hidden behind bureaucratic walls. Some labs resist sharing bench notes or raw data, claiming proprietary software or burden. Courts are uneven in enforcing transparency. A criminal defense lawyer has to be persistent and specific. You do not ask for “everything.” You ask for the analyst’s case file, the SOPs in effect at the time, validation studies for the relevant methods, instrument maintenance logs covering the case period, quality control charts, corrective action reports, and any communications that reflect analyst awareness of issues. If software was used, you ask for the project file and event logs.

That sounds like a slog, because it is. But once you have the materials, patterns appear. A lab that quietly changed a threshold mid-year. A flurry of corrective actions after an audit found cross-contamination in a drying cabinet. An analyst whose proficiency scores dipped. You do not accuse. You show.

Expert witnesses: choosing when to bring your own

Not every case needs a defense expert. Sometimes you can use the state’s analyst to build your doubt. Sometimes the budget says no, even if the science says yes. When you do bring an expert, choose carefully. You want someone who does not swagger, who can explain without condescension, who will call it against you if the data runs that way. Juries smell hired guns.

I once retained a DNA statistician who told me privately that the state’s likelihood ratio was reasonable given the lab’s assumptions. He still helped us by explaining, clearly, that “reasonable” did not mean “singular.” The jury learned that a likelihood ratio reflects competing models and assumptions. The verdict turned not on the DNA, but on shaky eyewitness identification. The expert’s transparency made us credible.

Cross-examination: questions that move needles

Good cross is a scalpel, not a hammer. You pick a few pressure points and make them simple.

What did you know about the case when you analyzed the evidence? Which steps in your method rely on your judgment rather than an algorithm? What validation studies support using this method on samples like this one? What quality controls were run in the same batch as this case, and how did they perform? If I changed this one assumption, what happens to your conclusion?

Those questions translate jargon into plain language. You do not need to prove the lab wrong. You need to show the path from “probably” to “definitely” crossed a rope bridge on a windy day.

When the science is truly strong

It is tempting to treat forensics as a field of sandcastles. That is not fair. Some results are robust. A single-source, high-template DNA profile with clean controls and a straightforward inclusion is powerful. A well-documented digital extraction that shows a device at a location while simultaneously sending messages that align with witness accounts deserves respect. A properly collected and confirmed drug sample leaves little room to argue.

In those cases, a criminal defense lawyer shifts strategy. You focus on the element the science does not speak to: intent, knowledge, identity in time, lawful authority, duress, a legal defense on the statute’s terms. You save your credibility for where it matters.

Lab culture and the long game

Individual cases matter. Lab culture matters more. When labs embrace transparency, blind proficiency testing, and error reporting without career suicide, everyone benefits, including prosecutors. Defense lawyers should applaud those moves and say so on the record. When labs dig in, hide behind proprietary claims, or punish internal critics, you document it and bring it to court.

I handled a case after a state lab had publicly acknowledged an analyst’s misconduct in unrelated matters. The prosecutor argued our analyst was clean. We were not alleging misconduct. We were showing that the lab’s oversight mechanisms failed for years, which made “trust us” less persuasive. The judge allowed broader cross, and the jury listened more carefully. Culture ripples.

The role of numbers and the seduction of certainty

Forensic disciplines sometimes present probabilities. Others pretend not to, while smuggling them in through language. A hair comparison that says “consistent with” is really trying to say “the probability of seeing this level of similarity given a random selection is not trivial, but given the suspect, it is higher.” Firearms, fingerprints, and toolmarks, if they avoid numbers, may still project categorical certainty. Numbers are not magic. They need context: database sizes, population structure, error rates in real-world conditions, and how the lab measured those.

When a statistic appears, my habit is to translate it to a sentence with both numerator and denominator. Juries handle that better than strings of zeros. A likelihood ratio of a billion does not mean a billion-to-one chance the defendant is guilty. It means the observed data is a billion times more probable if the defendant contributed than if a single unrelated random person did, under the model and assumptions used. If the model is shaky, that billion becomes a beautifully arranged pile of assumptions.

Practical realities: budgets, time, and triage

Public defenders do heroic work with thin resources. Appointed counsel juggle calendars. Private defense can help but carries its own constraints. A smart defense lawyer triages. Not every case needs a tour through every instrument log. You focus where the state’s case leans hardest on the lab. In a gun case with a shaky eyewitness, you might target the firearm toolmark testimony. In a drug case where the defendant is on video selling pills, you may accept the chemistry and fight the weight or the intent to distribute.

Clients often ask whether to spend money on an expert or on investigation. The answer depends on what the forensic evidence can actually prove. An unimpeachable DNA profile that puts the client in a car does not tell us whether he knew about the firearm under the seat. Maybe we spend on door-to-door canvassing instead. Judgment is not a science, but it improves with honest reflection about your past cases.

Stories that move courts

One of my earliest trials involved a burglary charge tied to a palm print on a broken window. The print was clear. The examiner was qualified. The prosecutor looked relaxed enough to nap. Cross revealed that the window belonged to a landlord who ran a cash business and that my client had done handyman work at the building for years. The examiner admitted a palm print cannot tell age in human time, only in lab time: the deposition happened at some point, not necessarily during the offense. The jury wanted a time machine. Since none was provided, they did what juries should do with uncertainty. They voted not guilty.

Another case revolved around mixed DNA on a steering wheel. The state leaned on a probabilistic genotyping output that included my client. The raw data showed low-level peaks in a noisy mixture. We retained a conservative expert who explained that, given the mixture’s complexity, the software’s output changed depending on the number-of-contributors assumption, which was not fixed by the data. The judge allowed us to explore those sensitivities. The jury did not pretend to be mathematicians. They heard that a machine gave different answers when asked slightly different questions. That made them cautious. Caution can save a life from a 20-year sentence.

The defense lawyer’s mindset: curious, skeptical, never cynical

Curiosity drives good forensic challenges. Skepticism keeps you honest. Cynicism gets you in trouble. Most analysts are trying to do it right. Most police are not out to frame your client. Most errors are the children of haste and habit. Treat people with respect, and they are more likely to share the uncomfortable details you need.

Rhetoric aside, here is a short checklist I keep near my desk for forensic-heavy cases. It is not a script, just a memory aid.

Identify the specific forensic claim in plain English and list the assumptions it requires. Obtain underlying data, bench notes, SOPs, validation, QC logs, and software details. Map chain of custody step by step and mark points of potential contamination or mix-ups. Decide whether you need a consulting expert or can do a surgical cross without one. Translate technical conclusions into risk statements a jury can understand, then test them against your case theory.

Keep the list short. Keep your mind open.

Why it matters beyond the verdict

Challenging forensic evidence is not about humiliating scientists or turning trials into tech theater. It is about aligning courtroom certainty with what the data and methods can honestly support. When the defense does that well, the system improves. Labs refine methods. Prosecutors adjust charging decisions. Courts shape standards that make future error less likely. And sometimes, an innocent person walks out of court and goes home, which is the point of the whole exercise.

I have seen jurors lean forward during a lab analyst’s testimony, spellbound by charts and acronyms. I have seen those same jurors nod when an analyst admits the limits of a method. That nod is trust forming in real time. The criminal defense lawyer’s job is not to break that trust. It is to build it the right way, by insisting that science be science, not theater, and that doubt be measured, not mocked.

Forensic evidence deserves respect and scrutiny in equal measure. If you do the work, if you honor the details, if you ask the plain questions at the right moments, you can meet the lab coat’s aura with something sturdier: clarity.

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Pub: 09 Oct 2025 07:27 UTC

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