DNA matches, fingerprints, blood evidence, and laboratory results can sound definitive when they are presented in a criminal case. Television and movies often reinforce the idea that forensic evidence provides a simple scientific answer: a sample is collected, a test is performed, and the person responsible is identified.
Real forensic investigations are rarely that simple.
Physical evidence must first be properly located and collected. It must then be preserved, transported, tested, interpreted, and connected to the circumstances of the alleged crime. Each step can affect what the evidence actually proves.
For defendants facing criminal charges in Los Angeles, forensic evidence may become one of the strongest parts of the prosecution’s case. It can also raise important questions about contamination, interpretation, chain of custody, laboratory procedures, and the conclusions offered by expert witnesses.
How Forensic Evidence Is Used in Criminal Cases
Forensic science applies scientific or technical methods to evidence connected with an investigation.
Depending on the allegations, investigators may examine:
- DNA
- Fingerprints and palm prints
- Blood and other biological material
- Firearms and ammunition
- Gunshot residue
- Drugs and controlled substances
- Toxicology evidence
- Hair and fibers
- Documents
- Digital evidence
- Toolmarks
- Other trace materials
Los Angeles County itself operates forensic laboratory services covering areas including toxicology, drug chemistry, DNA, gunshot residue, field criminalistics, and evidence control.
The significance of any forensic result depends on the particular case. DNA on an object may help establish that a person had contact with it, while fingerprints may support an argument that someone touched a particular surface.
But identifying biological material or a fingerprint is not necessarily the same thing as proving when it was deposited, why it was there, or what the person was doing at the time.
Those distinctions can become extremely important at trial.
DNA Evidence in Los Angeles Criminal Investigations
DNA evidence can be extraordinarily useful because, except for identical twins, individuals generally have different DNA profiles.
Biological material such as blood, saliva, semen, or skin cells may contain enough DNA for forensic scientists to develop a profile. That profile can potentially be compared with a known person’s sample or searched against qualifying profiles in DNA databases.
In some cases, DNA evidence can strongly associate a person with biological material recovered during an investigation.
However, the presence of someone’s DNA does not automatically establish exactly how or when that DNA reached a particular location.
That becomes increasingly important as modern testing methods become sensitive enough to detect extremely small amounts of biological material.
The Complications of Touch DNA
Investigators no longer need a large bloodstain or other obvious biological sample to potentially develop a DNA profile.
Modern forensic testing can sometimes detect DNA from a small number of skin cells left after someone touches an object. This is commonly called “touch DNA.”
That increased sensitivity creates powerful investigative possibilities, but it can also make interpretation more complicated.
The National Institute of Standards and Technology notes that small amounts of DNA may not necessarily be relevant to the crime under investigation. DNA can also be transferred between people, objects, and surfaces in ways that complicate conclusions about how it arrived at a particular location.
Consider an object that several people regularly handle. Detecting one person’s DNA on that object does not necessarily establish that the person touched it during the alleged crime.
Context still matters.
DNA Mixtures Can Be More Difficult to Interpret
A biological sample does not always contain DNA from only one person.
Evidence collected from clothing, weapons, vehicles, shared objects, or frequently touched surfaces may contain DNA from several individuals. These are known as DNA mixtures.
NIST’s 2024 scientific review of DNA mixture interpretation explained that mixtures can create challenges in determining how many people contributed DNA, separating individual profiles, evaluating trace amounts of material, and determining whether DNA is relevant to the event or may be the result of contamination.
The more complicated the mixture becomes, the more important the interpretation process may be.
Rather than asking only whether a defendant’s DNA appears somewhere in the sample, a defense attorney may examine:
- How many potential contributors were present
- How much DNA was recovered
- Whether the sample was degraded
- How statistical conclusions were calculated
- Whether alternative contributors could be excluded
- How the sample was collected and stored
- Whether contamination or transfer was possible
DNA evidence can be extremely powerful, but the scientific conclusion should still be considered in the context of the evidence actually tested.
California’s DNA Database
California maintains a statewide DNA and forensic identification database under the DNA and Forensic Identification Data Base and Data Bank Act.
California Penal Code Sections 295 through 300.4 establish the state’s DNA database framework, including provisions governing whose samples are collected, how profiles may be used, and circumstances in which qualifying individuals may seek expungement of DNA information.
DNA profiles may also be compared through the Combined DNA Index System, commonly known as CODIS.
A database hit can give investigators an important lead, but a database association does not eliminate the need to examine the underlying evidence.
Investigators and attorneys still need to consider where the biological material came from, the quality of the sample, how the comparison was performed, and how the DNA relates to the alleged offense.
Fingerprint Evidence Is Not Always a Perfect Print
Fingerprints have been used in criminal investigations for more than a century.
In an ideal situation, investigators might recover a clear print containing enough detail to compare with a known fingerprint. Crime scene evidence, however, is often much messier.
Latent prints recovered from surfaces may be partial, distorted, smudged, or affected by the material on which they were left. According to NIST, forensic fingerprint analysis involves determining whether a latent print contains sufficient detail and then comparing features in that print against a known sample.
That comparison includes human judgment.
Research has shown that trained fingerprint examiners can sometimes reach different conclusions when reviewing the same evidence. NIST has specifically studied human factors and potential sources of error in fingerprint examination.
Fingerprint evidence therefore should not automatically be treated as infallible simply because it is presented as forensic science.
What Does a Fingerprint Actually Prove?
Even an accurately identified fingerprint may answer only part of the question.
A fingerprint may support the conclusion that a person touched a surface at some point. It may not establish exactly when that contact occurred.
Suppose investigators recover a person’s fingerprint from an object inside a home the person regularly visits. That evidence may carry a different significance than the same person’s fingerprint appearing on an object they allegedly had no legitimate reason to encounter.
Location, timing, access, and surrounding circumstances all affect the meaning of the evidence.
For this reason, a Los Angeles criminal defense attorney may examine not only whether a fingerprint comparison is accurate but also whether the print actually supports the conclusion prosecutors are attempting to draw from it.
Collection and Preservation Can Affect Forensic Evidence
Before evidence ever reaches a laboratory, investigators must collect and preserve it.
Biological evidence can degrade. Trace material can be lost. Samples can potentially become contaminated. Packaging and storage conditions may also affect the condition of evidence over time.
NIST emphasizes that evidence management is a critical part of forensic science and that evidence should be protected from compromise, contamination, and degradation while its movement and handling are tracked.
That means defense review may begin at the crime scene rather than in the laboratory.
Questions can include who collected the evidence, whether gloves or appropriate collection tools were used, how items were packaged, whether samples were stored correctly, and how many people handled them before testing occurred.
Why Chain of Custody Matters
The chain of custody documents what happened to evidence from the time it was collected through testing and presentation in court.
For example, a piece of physical evidence might move from a crime scene investigator to an evidence facility, then to a forensic laboratory, then to an analyst, and eventually back into secure storage.
Records should allow those movements to be tracked.
Chain-of-custody questions can become important when there are unexplained gaps, unclear labeling, inconsistent documentation, or questions about whether the item tested was actually the same item recovered during the investigation.
A minor documentation issue does not automatically make forensic evidence inadmissible. However, the handling history may become relevant when deciding how reliable the evidence is and how much weight it should receive.
Contamination Can Change the Meaning of a Sample
Contamination is particularly important when dealing with modern DNA testing.
Because laboratories can detect increasingly small quantities of DNA, biological material accidentally transferred during collection, packaging, examination, or testing may potentially become part of a sample.
Contamination can occur at several stages.
An investigator may handle multiple pieces of evidence. Two items may be packaged improperly. Laboratory equipment or workspaces may introduce unwanted material if procedures are not followed correctly.
This is why forensic laboratories use protocols intended to reduce contamination and why defense attorneys may examine laboratory documentation when the origin or meaning of a DNA profile is disputed.
The question is not simply whether DNA was detected. It is whether the procedures used allow reliable conclusions to be drawn from that result.
Laboratory Procedures and Quality Control
Forensic laboratories generally operate under detailed procedures governing how evidence is received, tested, documented, and reviewed.
Those procedures can cover instrument calibration, reagent testing, controls, analyst qualifications, technical review, documentation, and methods for addressing unexpected results.
Los Angeles County, for example, reports accreditation for portions of its forensic laboratory operations under forensic testing standards.
Accreditation and standardized procedures can provide important safeguards, but they do not mean an error is impossible in a particular case.
When forensic evidence is especially important, a defense expert may review laboratory notes, testing data, quality-control information, analytical methods, and the interpretation provided by the prosecution’s expert.
Expert Witnesses and Forensic Evidence
Jurors ordinarily do not have specialized training in DNA interpretation, fingerprint comparison, toxicology, firearms examination, or other forensic disciplines.
That is why prosecutors and defendants may rely on expert witnesses to explain technical evidence.
California Evidence Code Section 801 permits qualified experts to offer opinions concerning subjects sufficiently beyond ordinary experience when expert testimony would assist the factfinder. The opinion must also be based on information of a type reasonably relied upon by experts in the relevant field.
An expert does more than simply announce a result.
The expert may explain the testing procedure, the data obtained, statistical calculations, limitations of the analysis, and what conclusions can reasonably be drawn.
The defense may also use its own expert to review the prosecution’s testing or provide another interpretation of the evidence.
New Scientific Techniques and California’s Kelly Standard
California courts apply additional scrutiny when a case involves certain new scientific techniques.
In People v. Kelly, the California Supreme Court adopted a general-acceptance standard for novel scientific methods. The purpose is to prevent a new technique from being presented to jurors with an unwarranted appearance of scientific certainty before it has achieved sufficient acceptance in the relevant scientific community.
The California Supreme Court continues to recognize the Kelly framework. When it applies to a novel scientific technique, the proponent generally must establish acceptance of the method in the relevant scientific field, appropriate expert qualifications, and proper use of the procedures involved in the particular case.
Not every form of forensic evidence requires a new Kelly hearing. Established techniques do not necessarily need to be repeatedly proven from the beginning in every prosecution.
However, emerging forensic technologies or new methods of interpreting evidence can raise additional admissibility questions.
Forensic Evidence Can Sound More Certain Than It Is
One of the challenges with scientific evidence is presentation.
Terms such as “match,” “consistent with,” “cannot be excluded,” or statistical probabilities can sound similar to someone unfamiliar with the underlying science, even though they may communicate very different conclusions.
NIST has emphasized that DNA evidence, particularly complex mixtures and trace samples, can be misunderstood when limitations and relevance are not communicated carefully.
The same principle applies more broadly to forensic evidence.
A scientific result should be understood for what it actually establishes rather than what someone assumes it proves.
For example, evidence showing that a defendant could be a contributor to a mixed DNA sample is different from evidence establishing that the sample came from only one person and matches the defendant’s profile.
Careful wording matters.
Reviewing Forensic Evidence in a Los Angeles Criminal Case
When forensic evidence plays a significant role in a prosecution, defense counsel may review materials including:
- Crime scene photographs and reports
- Evidence collection records
- Chain-of-custody documentation
- Laboratory reports
- Analyst notes
- Raw testing data
- DNA profiles and statistical calculations
- Fingerprint comparison documentation
- Instrument records
- Quality-control information
- Evidence packaging and storage records
- Expert reports
- Prior testing of the same evidence
- Alternative explanations for how biological or physical evidence arrived at the location
Independent expert review may also be appropriate in cases where the prosecution relies heavily on specialized scientific conclusions.
The goal is not to assume forensic evidence is inaccurate. It is to determine precisely what the evidence shows, whether appropriate procedures were followed, and whether the conclusions being presented are supported by the underlying science.
Los Angeles Criminal Defense and Forensic Evidence
DNA, fingerprints, and forensic laboratory results can be highly persuasive evidence, but they should not be treated as automatically conclusive.
Collection methods, contamination, DNA mixtures, laboratory procedures, human interpretation, chain of custody, and the context in which evidence was discovered can all influence what a forensic result actually means.
If DNA, fingerprints, or other forensic evidence plays an important role in criminal charges against you in Los Angeles, Simmons & Wagner can review how the evidence was collected, tested, interpreted, and presented and determine whether there are grounds to challenge its reliability, admissibility, or significance.
Contact Simmons & Wagner to discuss your case and learn more about your legal options.