Written by Virginia Barron | Criminal Defense Attorney & DNA Forensic Litigator
A contamination event does not define a laboratory. The response can.
A forensic DNA laboratory finds an unexpected DNA profile where no DNA should be. The obvious reaction is alarm. Was evidence contaminated? How many cases are involved? Can reported results still be trusted?
Those are important questions. But they are not the only questions.
On August 26, 2026, I presented at a contamination symposium for the Midwestern Association of Forensic Scientists with leadership from the Hennepin County Sheriff’s Office Forensic Science Laboratory. The audience was made up largely of forensic laboratory people. My portion of the symposium focused on what happens after a contamination event leaves the laboratory and becomes a legal issue: what attorneys need to know, what disclosure requires, what preserves credibility, and how the event can change litigation strategy.
The symposium grew out of a real contamination event at the Hennepin County Sheriff’s Office laboratory. And one of my strongest takeaways from that event was not that contamination occurred. It was how the laboratory responded.
A contamination event is not automatically evidence that a laboratory failed. In many cases, discovering contamination is evidence that the quality-control system worked. What matters next is how completely the laboratory investigates, documents, communicates, and limits its conclusions.
That distinction matters to forensic scientists. It matters to prosecutors. It matters to defense attorneys. And most importantly, it matters to the people whose cases depend on the evidence.
What Happened at the Hennepin County Forensic Science Laboratory?
In September 2024, the Hennepin County Sheriff’s Office Forensic Science Laboratory halted DNA testing after an unexpected DNA profile appeared in a negative control. The laboratory later explained in a public media release that its scientists traced the profile to a plastic plate supplied by a third-party vendor and used in the DNA-testing process. The same unknown profile had also been observed by laboratories elsewhere in the United States and internationally.
The response was extensive. HCSO reported reviewing more than 50,000 samples from prior casework, eliminating the product from use, testing a replacement product, and recommending fewer than ten cases for retesting. The Minnesota Star Tribune reported that the DNA section was shut down for 79 days while analysts investigated the source, reviewed years of prior profiles, and completed corrective work before resuming casework.
The fact that the contamination appeared in a negative control is important. A negative control exists, in part, to reveal DNA that should not be present. In other words, the laboratory’s quality system did what it was designed to do: it generated a warning that something had entered the process that did not belong there.
HCSO’s own public statement made the same point: the risk of contamination can be reduced, but it cannot be scientifically eliminated altogether, which is why forensic laboratories use controls and other quality procedures. The laboratory is an ANAB ISO/IEC 17025 accredited forensic testing laboratory that provides DNA and other forensic services to law-enforcement agencies in and around Hennepin County.
I told the Minnesota Star Tribune that the laboratory had gone “above and beyond” in its communication. I still believe that. From a defense perspective, this is the kind of investigation and documentation I want to see whenever a significant forensic quality issue is discovered. The way the HCSO lab handled this situation should be used by other labs as a blueprint for how to handle the situation correctly and with scientific integrity.
A Laboratory Question and a Legal Question Are Not the Same Question
One of the central points of my presentation was that a contamination event produces two different questions. They overlap, but they are not interchangeable.
The laboratory question
The legal question
Did we detect it, contain it, investigate it, and correct it?
Does the event change the evidentiary value of the DNA result in this case, or in other cases?
A laboratory may answer the first question extremely well. It may locate the source, remove it, validate replacement equipment or consumables, identify the relevant time period, and determine what corrective action is scientifically appropriate.
But an attorney may still need different information. Did the contaminant appear in the evidence sample? Was the sample a complex low-level mixture? Did the unexpected alleles overlap with a known contributor? Was the profile already interpreted, compared, uploaded, or testified about? Was there enough remaining extract to retest? When did the laboratory learn of the issue, and when did the parties learn about it?
The same contamination event can have very little legal significance in one case and enormous significance in another. That is why neither “there was contamination” nor “the laboratory fixed it” answers the case-specific legal question by itself.
Contamination Does Not Mean Every DNA Result Is Wrong
It is easy to overstate a contamination event. That is no more scientifically responsible than minimizing one.
A contaminant detected only in a control, clearly identified through an elimination profile, with unaffected case data and a reproducible retest may have little effect on the evidentiary value of a particular result. On the other hand, a low-template mixture with limited remaining sample, uncertain contributor number, allele overlap, or a result that was already relied upon in litigation may require much closer review.
The question is not whether the word “contamination” appears somewhere in a laboratory record. The question is what happened to this sample, this profile, this interpretation, and this case.
Overstating a contamination event damages a laboratory unfairly. Understating one damages a defendant unfairly. Neither serves the criminal justice system.
What Impressed Me About the HCSO Response
I am a defense attorney. My job requires me to scrutinize forensic evidence, challenge unsupported conclusions, and ask questions that laboratories and prosecutors may not want to hear. That does not mean I assume every laboratory problem proves misconduct or incompetence.
The HCSO event is a good example of why that distinction matters. The laboratory did not stop at identifying an unexpected profile. It shut down DNA casework, worked to trace the source, manually reviewed a massive body of prior casework, replaced the implicated product, tested the replacement, and communicated with criminal-justice stakeholders while the investigation was unfolding.
That matters because uncertainty is unavoidable during an active quality investigation. The laboratory may know that a problem exists before it knows precisely how far the problem extends. Waiting for perfect certainty before communicating can create a second problem: attorneys may make plea, trial, expert, or sentencing decisions without information they would have considered important.
A careful disclosure can say: here is what we know, here is what we do not know yet, here is what we are testing, and here is when we expect to know more. That is more useful than silence followed by a final conclusion months later.
Ambiguity does not reduce litigation. Ambiguity generates litigation.
A vague notice forces attorneys to reconstruct the event through discovery motions, subpoenas, hearings, and cross-examination. A detailed chronology and a clear description of scope allow both sides to focus on the cases that actually require attention.
What Attorneys Need When a DNA Laboratory Discovers Contamination
When a significant contamination event touches casework, attorneys need enough information to evaluate the event independently. In my presentation, I identified several categories that are particularly important:
Plain-language description of what happened and how it was discovered.
The scope of the event: affected samples, cases, profiles, instruments, consumables, staff, dates, or workflows.
A clear separation between what is confirmed, what remains a working hypothesis, and what is still unknown.
Whether results were reported, compared, uploaded to a database, relied upon in charging or plea decisions, or previously presented in court.
Corrective actions taken, including any retesting or reanalysis options.
Any limitations on interpretation going forward and the basis for concluding particular samples or cases were unaffected.
A usable timeline showing discovery, investigation, confirmation, corrective action, and disclosure.
The goal is to understand the event well enough to distinguish cases where it is largely irrelevant from cases where it changes the evidentiary picture.
Why Disclosure Timing Matters
A laboratory is not expected to decide whether a contamination event changes a plea offer, creates impeachment value, affects an admissibility motion, or alters defense strategy. Those are legal judgments. That is precisely why the underlying facts matter.
Minnesota’s discovery rules require disclosure of scientific test results and reports relating to the case, exculpatory information within the prosecutor’s possession and control, and other discoverable case-related material. Minn. R. Crim. P. 9.01 also extends the prosecutor’s obligations to certain people who participated in the investigation or evaluation of the case and who report to the prosecutor’s office. Whether a particular laboratory or outside agency falls within that scope depends on the actual relationship and role in the case.
Rule 9.03 further requires disclosure in time for counsel to make beneficial use of the information and imposes a continuing duty to disclose newly discovered material before and during trial. The rule’s timing language matters because eventual disclosure is not always equivalent to useful disclosure.
Constitutional disclosure principles may also come into play. Brady v. Maryland addresses favorable material evidence; Giglio v. United States extends the disclosure obligation to impeachment information; and Kyles v. Whitley explains that the prosecutor’s responsibility can include favorable evidence known to others acting on the government’s behalf in the case. The legal significance depends on the information, the relationship of the laboratory to the prosecution, the timing, and the effect on the defense.
The practical point is simpler: attorneys cannot evaluate information they do not know exists.
Transparency Protects Laboratory Credibility
One of the easiest mistakes after a contamination event is to assume that transparency will make the laboratory look worse. In my experience, the opposite is often true.
Credibility is strengthened when the laboratory promptly documents the event, distinguishes facts from assumptions, identifies the boundaries of the investigation, corrects reports or testimony when necessary, and acknowledges uncertainty without defensiveness. Credibility is damaged when explanations change without documentation, conclusions are offered without showing how they were reached, or the laboratory appears to minimize the issue before the investigation is complete.
A forensic scientist who can say, “Here is what happened. Here is how we know. Here is what we did to test that explanation. Here are the cases we believe were affected. Here are the limits of that conclusion,” is a far more persuasive witness than one who simply insists that everything is fine.
The limit is often the most important part. A witness who explains what was not tested, what was not reviewed, and what remains uncertain gives the fact-finder a reason to trust the conclusions that are supported.
A DNA Profile Case Can Become a Laboratory-Process Case
Once a contamination event becomes relevant to a criminal case, the litigation often changes. The focus may expand from the profile itself to the laboratory process that produced it.
That can affect expert review, discovery, admissibility, cross-examination, plea negotiations, trial strategy, and sometimes whether a result should be retested or reinterpreted. Quality-assurance records, corrective-action documents, validation materials, internal communications, and the timeline of the investigation may become relevant in ways they were not before.
This does not mean every contamination event justifies a broad attack on the laboratory. It means the defense must understand the process well enough to know whether the event changes anything material about the evidence being used against the client.
That same principle runs throughout my DNA work. A result can be scientifically accurate and still be legally misleading if it is presented without the context necessary to understand what the science can and cannot establish. The same is true when evaluating STRmix and DNA mixture interpretation, or when deciding whether DNA on an object proves direct contact rather than secondary or tertiary transfer.
What a Good Laboratory Response Looks Like
A good laboratory is not necessarily a laboratory in which nothing ever goes wrong. Forensic science is performed by people using instruments, software, reagents, consumables, databases, and complex workflows. Quality systems exist because errors and contamination risks cannot be reduced to zero.
What matters is what happens next.
Detect it.
Contain it.
Investigate it.
Document it.
Define what is known and what remains unknown.
Disclose it early enough for the parties to use the information.
Correct what needs to be corrected.
Do not claim more certainty than the investigation supports.
That kind of response does not weaken confidence in forensic science. It is how confidence is earned.
When DNA Evidence Is Part of a Minnesota Criminal Case
A contamination notice does not tell you, by itself, whether the DNA evidence in a particular case is reliable or unreliable. The answer depends on the sample, the profile, the laboratory process, the interpretation, the timing, and what the underlying records show.
When I review DNA evidence for a criminal case, I look beyond the final laboratory report. I want to understand the underlying data, the laboratory’s procedures, the assumptions made during interpretation, quality events that may affect the analysis, and whether the testimony accurately reflects the limits of the science.
Barron Law Office represents people facing criminal charges in southwestern Minnesota and provides forensic DNA consultation to defense attorneys throughout Minnesota. If DNA evidence is playing a significant role in your case, call Barron Law Office at 507-822-5735 to talk about whether an independent review may be appropriate.
Frequently Asked Questions About DNA Contamination in Forensic Laboratories
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DNA contamination occurs when DNA that is not part of the evidentiary sample enters the testing process or appears in the resulting data. The source can be a person, another sample, the laboratory environment, a reagent, or even a consumable used during testing. The HCSO event is an example of contamination traced to a third-party plastic plate used in the testing process which was contaminated well before it ever got to the lab.
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No. The legal and scientific impact is case-specific. A contamination event may have little effect on one sample and substantial effect on another. The important questions include where the contaminant appeared, the complexity and quantity of the DNA, whether interpretation was affected, whether retesting is possible, and what the laboratory’s investigation established.
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An unexpected DNA profile appeared in a negative control of one analyst, who brought it to their supervisor. The supervisor then saw the same contamination profile in a completely separate batch. Negative controls are designed to reveal contamination or other unexpected DNA in the testing process. The laboratory halted DNA casework and investigated until it traced the recurring profile to a plastic plate supplied by a third-party vendor. During that process it was learned that this same contamination profile had appeared in multiple state labs throughout the United States and international labs overseas. This profile had been uploaded to databases and had been around the scientific community for years. Based on the thorough investigation done by the HCSO Crime lab it had been in the forensic science community for approximately nine years that it was able to trace.
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Halting casework can preserve the integrity of the investigation and prevent the same unknown source from continuing to enter new testing. In Hennepin County, the laboratory paused DNA analysis while it investigated the source, reviewed prior casework, changed products, and tested the replacement before resuming operations.
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Yes. A contamination event can still matter to foundation, impeachment, disclosure, expert review, or the explanation of how the result was generated. Whether it matters legally depends on the facts of the case. An unchanged conclusion does not automatically make the underlying quality information irrelevant.
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At minimum, the attorney should understand what happened, how the event was detected, the affected time period and workflow, which samples or cases were reviewed, the laboratory’s corrective actions, retesting options, the basis for any conclusion that the client’s result was unaffected, and the timeline of disclosure. Depending on the case, quality-assurance and corrective-action records may also require review.
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The prosecutor carries the discovery obligation in a criminal case, and the precise scope of information attributed to the prosecution depends on the governing discovery rules and the relationship between the laboratory and prosecutor. Minnesota Rule of Criminal Procedure 9.01 extends the prosecutor’s obligations to certain people who participated in investigation or evaluation and reported to the prosecutor.
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Because defense counsel needs time to make meaningful use of the information. A contamination issue discovered before a plea, expert deadline, admissibility hearing, or trial can affect decisions very differently than the same information disclosed after those decisions have already been made. Minnesota Rule of Criminal Procedure 9.03 requires timely disclosure and imposes a continuing duty before and during trial.
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A strong response detects and contains the problem, investigates its source and scope, documents the reasoning, distinguishes confirmed facts from assumptions, communicates uncertainty, identifies affected casework, takes corrective action, and explains the limits of its conclusions. Transparency allows attorneys and courts to evaluate the event without guessing. This is exactly what the HCSO lab did, and they did it well.
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Independent review is worth considering when the DNA evidence is important to the prosecution’s theory, the sample is low-level or mixed, the event could affect interpretation, retesting is limited or impossible, the contamination was discovered after the result had already been relied upon, or the laboratory’s explanation leaves important unanswered questions.
Primary Sources and Authorities
• Hennepin County Sheriff’s Office, Forensic Science Laboratory
• Minnesota Rules of Criminal Procedure, Rule 9.01 and Rule 9.03
• Brady v. Maryland, 373 U.S. 83 (1963)
• Giglio v. United States, 405 U.S. 150 (1972)
• Kyles v. Whitley, 514 U.S. 419 (1995)
• State v. Hunt, 615 N.W.2d 294 (Minn. 2000)
• State v. Roan, 532 N.W.2d 563 (Minn. 1995)
This article also draws from Virginia M. Barron’s August 26, 2026 presentation, “Legal Impact and Response: A Contamination Event After It Leaves the Laboratory,” at the Midwestern Association of Forensic Scientists contamination symposium.
This article provides general information about forensic DNA evidence and Minnesota law. It is not legal advice for any particular case.