Evidence and Testing

Prosecuting a case that contains DNA evidence is a challenge that requires thorough knowledge of rights, science and precedents.

Crime scene technicians and law enforcement may have collected numerous items of evidence relevant to the case that contain biological material from one or multiple sources. Prosecutors will need to have a complete understanding of the potential for DNA testing of crime scene evidence. A close relationship with crime laboratory personnel is the best way to facilitate an understanding of how DNA evidence might impact the case.

Statutes and Precedents

Jurisdictions have different statutes of limitations that may prevent or hinder the prosecution of suspects identified through DNA databases. Some jurisdictions have either eliminated or extended statutes for specific crimes such as sexual assault (the crime involved in this case study).

Case law is continually developing as the science evolves. For example, courts have held that DNA evidence is legally sufficient to convict even without identification of the accused by a live witness.

In some jurisdictions, after a defendant has been charged, the prosecution is required to preserve evidentiary biological material for the purpose of DNA testing by the defense.

Furthermore, the existing biological material may be too small to conduct two separate tests (one for the prosecution and one for the defense). Prosecutors should consult with the laboratory on the feasibility of dividing the sample. In these circumstances, the state may require prosecutors to inform the defense that the sample is too small and to seek the court's permission to consume the entire sample.

Meet With the Experts

As early as possible, prosecutors should meet with the laboratory personnel who have performed the DNA testing in the case.

  • Review their reports and case files
  • Ascertain and discuss any anomalies that may be present
  • Review the chain of custody
  • Review the work of the defense laboratory (if the defense has conducted additional testing)

Consult with the laboratory to determine the manner in which evidence was collected and stored, and to ascertain whether conditions could have led to degradation or contamination.


Anticipate information the defense will need and prepare to provide it when it is formally requested in discovery.

Together with DNA laboratory personnel, ensure that all discoverable materials, including reports, notes, chain of custody documentation, written correspondence and results are available to the defense (preferably in electronic file format).

In cases where the defense might present DNA evidence, the prosecution may file a discovery request as well.

Specific discovery requests may include: validation studies, proficiency test results, contamination incidents and corrective actions, and accreditation status. Together with the DNA laboratory personnel, review the results of the defense laboratory's tests. Identify discrepancies or problem areas. Additional DNA testing may be requested by the prosecution (discussed later in this module).

Determine What Was Collected and Tested

In consultation with the laboratory, it must be determined if probative evidence exists that was never tested for DNA. The prosecutor will then decide whether or not such evidence should be tested and presented at trial.

In the case study crime, the prosecution will assess whether evidence located in other parts of the house was collected but not tested such as bed linens, the victim's clothing and undergarments, or blood retrieved from windows or doors.

Plan for New Testing

Additional DNA testing with newer technology may yield more informative results, particularly in cold cases. Consultation with the laboratory will determine if this would be beneficial.

The prosecution need not, and should not, test every item from the crime scene as many items will have no evidentiary value. The determination of which additional items should be tested is best made in consultation with investigators and laboratory personnel.

Obtaining and Testing New Reference Samples

DNA cases may require the obtaining of reference samples to explain the presence of extraneous DNA profiles. Reference samples are usually obtained from the individual by consent, but can be obtained by court order, if necessary.

For example, in the case study, the defense claimed that another perpetrator (not the suspect) could have committed the crime because the bed linens contained a semen stain that did not match the victim or the suspect. It was critical to the prosecution's case to obtain reference samples from individuals who had used the same bed (where Sandy was found murdered) to refute the defense claim. Elimination samples were obtained and it was determined that the extraneous DNA belonged to a relative who had previously stayed in that room when he was visiting Lucy's family.

Testing of emergency personnel and scientists may be also helpful to help the laboratory identify and explain the presence of extraneous DNA profiles contained in crime scene evidence.

Communication with the Laboratories

Prosecutors should have a good understanding of the volume of work forensic DNA laboratories manage; cases must necessarily be prioritized. If a case is resolved (pled out, for example) and further analysis is not needed, the laboratory should be informed as soon as reasonably possible. Prompt notification to the laboratory will enable the laboratory to move on to the next case, saving valuable time and resources.

Motion in Limine

A motion in limine is a request to bar, limit or, in some cases, admit specific pieces of evidence. The defense in the case study will move in limine to preclude mention of the fact that the accused was arrested after the DNA from the crime scene was found to match his DNA profile in CODIS.

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Motion to Suppress

A motion to suppress seeks to exclude evidence seized in violation of constitutional (or sometimes statutory) rights. Challenges can be brought when the defendant's DNA was obtained through an unconstitutional search and seizure.

Issues may include the validity and scope of consent, the lawfulness of a stop or seizure, and whether a warrant was required.

Voir Dire

The extent and scope of voir dire will depend on the individual jurisdiction and the particular trial judge. When conducting voir dire in a case involving DNA evidence, the prosecution should explore areas of inquiry that will enable it to gauge a potential juror's educational background and understanding and comfort level with scientific concepts. Understanding the jury panel's level of education and scientific expertise will assist the prosecution in tailoring its presentation to maximize the jury's ability to understand the expert's testimony.

Depending on the rules of the jurisdiction, prosecutors may wish to consider the following voir dire questions:

  • Whether any of the panel members have particular training, education, or experience in biology, chemistry or statistics
  • Whether any of the panel members have read books, articles or magazines on DNA analysis methods
  • Whether any of the panel members would be unable to consider the case fairly because of something they have seen in the media or on television shows such as CSI or Law and Order
  • Whether there is anyone on the panel who believes that the techniques they have seen on television are techniques that can always be performed by any law enforcement agency in a real case.
  • Whether any of the panel members have strong opinions for or against DNA. This question is important to filter out skeptics who do not believe that DNA is reliable.

In jurisdictions that permit extensive questioning of potential jurors, the prosecution may also use the voir dire process as an opportunity to educate the jury panel about DNA evidence.

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Opening Statement

During the opening statement, the prosecutor should focus on the facts - stating where the victim's blood was found, where blood matching the defendant's DNA profile was found, etc. Carefully preview the DNA evidence and the expert who will testify, making sure not to overwhelm the jury with too much detail. It is also essential that the prosecutor refrain from misstating or overpromising what the DNA evidence will show in the case. The prosecution may wish to consult with its expert regarding how to properly describe the DNA analysis results during the opening statement.

Direct Examination

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Presentation of Evidence

As an initial matter, the prosecutor should present testimony regarding how the evidence was collected, packaged and stored. Typically, a crime scene officer will be called to provide such testimony. The testimony should focus on the steps that were taken to guard against contamination of the crime scene and the evidence. The prosecutor should review each item of evidence with the crime scene officer, eliciting how the evidence was collected, packaged and stored; demonstrating proper chain of custody; and accounting for all of the evidence. When possible, use photographs and actual items of evidence during the officer's testimony. Ask the officer about problem areas to diffuse the potential impact of cross-examination.

The prosecution should meet with its DNA expert well in advance of trial to prepare for the presentation of the DNA analysis results and to discuss potential issues that might be brought by the defense. Be prepared to address issues relating to cold cases, such as missing or damaged evidence and unavailability of witnesses within the chain of custody.

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Expert Witness Qualification

Prosecutors may be required to qualify their witness in the area of forensic DNA analysis. Once the proper foundation has been established by eliciting the witness qualifications, proffer the expert to the court. Depending on the jurisdiction, the court may at that point read an instruction regarding expert witnesses.

Effective expert witness qualification will include highlighting the witnesses' education, training and hands-on experience. The witness should be asked about participation in continuing education programs, professional conferences, advisory boards and teaching experience. If applicable, the prosecution should also highlight any scientific articles on DNA or related scientific concepts published by the witness. Although the defense may stipulate to the credentials of the witness, the prosecution should be as thorough as possible in explaining the credentials and experience of the witness for the benefit of the jury.

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Expert Testimony

Once the expert has been qualified, prosecutors should consider following the strategy of primacy and recency. The prosecution would begin by asking the witness if DNA analysis was conducted on items of evidence, whether or not DNA profiles were obtained, and whether any of the DNA profiles obtained from the crime scene evidence matched that of the defendant. The prosecution should then have the expert describe DNA and the basic principles of forensic DNA testing. Prosecutors can also include questions regarding the application of the underlying methodologies for DNA analysis to other scientific fields such as the matching of organ donors for transplants or identifying victims of mass disasters.

Prosecutors should consult with their expert regarding the use of visual aids, such as charts depicting the electropherograms generated as a result of the DNA analysis. To maintain the jury's attention, prosecutors should consider having the expert integrate his or her findings with exhibits depicting crime scene photographs of bloodstains or clothing items. When the results involve mixtures or are inconclusive for some items, be sure to discuss in advance the best way to present testimony about these issues. Prosecutors should cover problem areas that defense counsel are expected to address during cross-examination.

Once the expert has described a DNA match, prosecutors should ask him or her about the statistical significance of that match. This is frequently the most complicated aspect of the expert's testimony; therefore, prosecutors should review statistical information thoroughly with the expert.

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Redirect provides the opportunity for the prosecution to clarify issues that may have been brought by the defense during its cross-examination of the witness. A well-prepared defense attorney who understands the scientific concepts of forensic DNA analysis can give the appearance that he or she is exposing problems with the DNA analysis. As a result, the prosecution must be prepared to rebut the points raised by the defense during cross-examination.

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Prosecutors should listen intently to the defense cross-examination. The best redirect is brief and focused. Prosecutors should make sure to ask the expert to clarify points or address erroneous assertions made by the defense during cross.

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Some cases might require cross-examination of the defense expert; preparation is key in these instances. If reports have been written by the defense expert, it is very important that the prosecution review them in consultation with its own expert. In advance of trial, it might also be helpful for the prosecution to obtain transcripts of cases in which the expert has previously testified. In extreme cases, the prosecution may wish to file a motion in limine to preclude the defense expert's testimony if it can be demonstrated that the expert lacks the necessary qualifications or will testify to a method or principle that lacks reliability.

If the expert has solid credentials, prosecutors should begin by getting the expert to agree that the underlying techniques of forensic DNA analysis are reliable and widely used by the scientific community. As much as possible, prosecutors should try to get the defense expert to state that he or she agrees with the methodologies and techniques used, and that the quality assurance and quality control measures that accompany forensic DNA are stringent.

If the defense expert lacks hands-on experience, prosecutors should highlight this during cross-examination. It should also be noted if the expert has not published articles or conducted any research in the underlying scientific techniques of forensic DNA analysis. Many defense experts make a substantial living above and beyond their "day jobs" by testifying. Others may have never worked in a forensic setting. Where appropriate, prosecutors should expose these facts during cross-examination.

Often defense experts have not examined the actual results in a particular case. In such cases, the prosecutor may wish to cross-examine the expert on how he or she is able to point out mistakes without prior review of the results. Also, if the defense expert attacks laboratory protocols or discusses "error rates," the prosecutor may be able to ask about the defendant's ability to conduct independent DNA tests in a particular case. Research the law of the jurisdiction before doing so and obtain clearance from the court to avoid shifting the burden of proof.

Sometimes the prosecution will be faced with a defense expert who calls into question the reliability of the underlying science or the statistics used in forensic DNA applications. In such situations, the prosecution may need to be prepared to call its expert in rebuttal.

Closing Argument

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The closing argument should summarize how the DNA evidence links the defendant to the crime. The amount of time the prosecutor devotes to summarizing the DNA evidence will depend on how central the DNA evidence is to the case.

The prosecutor must correctly state the statistical conclusions that accompany the DNA analysis results in the case. Misstatement of the statistical evidence can result in reversal of a conviction. Oftentimes, prosecutors paraphrase an expert's statistical conclusions and accidentally mistake the meaning of the statistical frequency results. This is referred to as the "prosecutor's fallacy." To avoid this, prosecutors should ask their expert what language to use when describing his or her statistical conclusions and stick to this throughout the trial. Prosecutors should refrain from "getting creative" with the statistical results during closing argument. Prosecutors should be ready to object during the defense closing argument if the attorney misstates the expert's statistical conclusions.

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Finally, if the defense did not have access to their own expert and/or laboratory for testing of biological evidence, prosecutors should be careful not to refer to this fact as it might constitute burden shifting. A strong closing argument will include the restating of facts and a summary of evidence presented to support those facts, in addition to comment on the defense theory and a request for a judgment or verdict.

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Cases involving DNA evidence can present issues beyond those of a typical criminal case. A thorough understanding of the issues and considerations associated with prosecuting DNA cases provides a solid foundation throughout all trial phases.

As illustrated in this case study, DNA evidence can present scientific and legal complexities that have a profound impact during trial preparation and courtroom proceedings. A thorough understanding of scientific DNA analysis methods aids in determining the significance and value of biological evidence. Understanding legal issues that may be encountered when presenting case-related DNA evidence is helpful in developing effective courtroom strategies. Through practical examples presented in this case study, officers of the court can gain valuable insight into the powerful role DNA evidence plays in prosecuting the guilty and exonerating the innocent.