By Dr. Matthew Loux and Bryce Loux | 08/28/2025
The prosecution and defense rely on evidence for building their arguments in court and ensuring fair outcomes. When law enforcement personnel preserve evidence, it supports claims by the prosecution or defense, establishes timelines, and ultimately determines guilt or innocence.
If evidence is not managed properly or goes missing, it can obstruct investigations or even result in wrongful convictions or acquittals. The duty to preserve evidence goes beyond following policies or procedures – it is a constitutional obligation rooted in due process.
Why Is It Important to Preserve Evidence?
From a constitutional perspective, the preservation of evidence relates to due process. Each defendant has the right to a fair trial, including the ability to challenge related evidence presented against them.
In Brady vs. Maryland, for example, the Supreme Court held that the suppression of evidence favorable to an accused perpetrator violated due process, reinforcing the state’s burden to keep and disclose materials that could clear the accused. This duty applies to only government actors, including prosecutors, police, and state investigative agencies.
The ability to appeal or seek post-conviction relief often depends on whether crucial materials – such as DNA evidence or other potentially exculpatory items – are available. Over time, DNA and other forensic evidence has exonerated hundreds of wrongfully convicted prisoners. If police officers do not preserve evidence properly, that can result in permanently losing the ability to review or reverse a conviction.
Loss or improper storage of such evidence undermines public trust and confidence in the justice system. High-profile, missing evidence scandals have overturned wrongful convictions and driven reforms in law enforcement’s actions and evidence handling policies nationwide.
Types of Evidence in Criminal Cases
Police typically collect one or more types of evidence in a criminal case, so law enforcement officers must be diligent when they collect and preserve evidence. This evidence falls into four primary categories, and each requires different techniques and methods to retain its integrity.
The four primary categories are:
- Physical evidence
- Biological evidence
- Digital and electronic evidence
- Testimonial and documentary evidence
Physical Evidence
Items like weapons, clothing, and fingerprints are tangible links to a crime scene. These materials must be labeled, packaged in suitable containers, and stored in secure environments to eliminate any risk of contamination. Law enforcement typically uses chain-of-custody documentation to reinforce the integrity and authenticity of this type of evidence.
Most evidence often undergoes laboratory analysis, where investigators examine blood samples collected by technicians, review tool marks, or analyze fibers. They also review ballistic evidence and fingerprints. In some cases, forensic firearms examiners determine if spent projectiles and cartridges were fired from a specific firearm that may have been used as a murder weapon.
In addition, fingerprint analysis can identify individuals who have had physical contact with a particular item or location. For results to be reliable, however, forensic laboratories performing the work must meet accreditation standards and follow local and state rules for admissibility.
The techniques for collecting and preserving this type of crime scene evidence include:
- Photographing and documenting the evidence before removal
- Packaging evidence separately to avoid cross-contamination
- Using evidence bags, boxes, and labels for clarity
- Employing gloves and tweezers for trace evidence
There are also legal considerations to collecting this type of evidence, such as:
- Improper labeling or storage can cast doubt on the authenticity of evidence.
- Evidence must be tested and handled by forensic technicians.
Biological Evidence
Biological evidence refers to body fluids like blood, saliva, and semen, as well as hair and other parts of the body that contain DNA. DNA evidence is very fragile, so it requires strict protocols to prevent degradation, such as:
- Rapid processing
- Cold storage
- Sterile packaging
- Prompt processing
To prevent cross-contamination and the loss of genetic material, law enforcement organizations must spend money on climate-controlled storage. Additionally, the use of masks, gloves, and sterile tools is essential to prevent contamination, and preserving evidence begins at the moment it is collected.
The collection techniques for biological evidence involve:
- Sterile gloves and tools to prevent evidence contamination
- Breathable containers like paper envelopes to prevent mold from developing on the evidence
- Accurate labeling with details such as the collection date, time, and location
Legal considerations for this type of evidence include:
- Maintaining a proper chain of custody to ensure the evidence’s admissibility in court
- Avoiding any mishandling or degradation to prevent the evidence from being destroyed
Digital and Electronic Evidence
In today’s world, digital evidence can be crucial in investigations. This type of evidence includes:
- Emails
- Emergency call recordings
- Texts
- Social media posts
- Mobile phone data
- Audio or video recordings
However, digital evidence is vulnerable to various issues, such as deletion, encryption, and hardware failures. For digital evidence to be useful in an investigation, it must be collected and stored promptly. As a result, up-to-date collection tools are essential.
Specialized skills and tools are required to interpret electronic evidence. Digital forensic examiners must maintain the original data in its unblemished, unaltered state, and they may only analyze verified copies. The evidentiary value may be compromised if there is any alteration, evidence tampering, or mishandling.
When digital and electronic evidence is collected for preservation, the collection techniques include:
- Isolating devices from networks to prevent remote data wiping, tampering, or alteration
- Remotely disabling lost or stolen devices
- Cloning hard drives to retain data integrity
- Collecting metadata
- Storing encryption keys and passwords securely
- Documenting software and hardware configurations
There are also legal considerations in regard to this type of evidence:
- For the evidence to be admissible in court, law enforcement agencies must use proper warrants and comply with privacy laws.
- Law enforcement personnel must maintain a proper chain of customer as well as detailed access and duplication logs.
Testimonial and Documentary Evidence
Testimonial and documentary evidence typically includes:
- Witness statements
- Police reports
- 911 call logs
- Written confessions
These types of evidence must be accurately documented and archived. Failure to do so may result in a loss of context or a legal advantage to the opposition.
Investigators should preserve investigative notes and includes witness statements in both physical and digital formats. These different formats ensure that the evidence is easily available for appeals or retrials.
Sound legal procedures must be followed when police officers collect testimonial evidence. For example, suspects must be advised of their Miranda rights during collection.
Also, interviews should occur in controlled environments that allow investigators to tape record statements under lawful conditions. This strategy protects a defendant’s rights and reduces the risk of an alleged violation or violation-compromised claims.
The collection techniques for testimonial and documentary evidence include:
- Using structured, clear audio/video environments for interviews
- Employing certified transcription services for accuracy
- Recording accurate interview details, including time, place, and participants
- Following protocols that ensure voluntary cooperation
- Avoiding coercion that could limit testimony in the future
Legal considerations for this type of evidence include:
- Obtaining inconsistent or poorly documented statements cause a risk of losing credibility.
- Hearsay, bias, or competency issues can challenge evidence admissibility.
Chain of Custody and Documentation
Maintaining a chain of custody through proper documentation is perhaps the most important task of law enforcement. It involves maintaining specific documented records of exactly what evidence is preserved, starting from collection to the point of submission in court.
The chain of custody must include the name, date, time, and location of each transfer, along with the reason for the transfer. These records help prevent any claims of tampering or contamination while ensuring accountability. In cases where a prosecutorial authority seeks to admit evidence, these records become essential for countering claims of excluding evidence due to mishandling.
Proper chain of custody procedures include:
- Labeling evidence with tamper-evident tape
- Maintaining logs with evidence management systems
- RFID tracking
- Barcodes
- Audited access
A broken chain of custody can render evidence inadmissible in court, which significantly weakens a case. Defense attorneys carefully inspect records to identify any mishandling that may dispute the claims made by the prosecution or show that a government agency acted improperly.
Modern methods of maintaining chains of custody tend to utilize technology so that the evidence can be tracked in real time and any tampering causes an alarm to be set off automatically. Through tools, human error is minimized and accuracy is maximized.
Common Problems with Evidence Preservation
Regardless of laws and procedures, many factors pose a risk to the integrity of evidence, including:
- Oversight and carelessness
- Limited storage space and strict budgets
- Technological challenges
Oversight and Carelessness
Improper labeling, storage, or restricted access can occur because of people’s forgetfulness or insufficient training. These mistakes are common when there are weak frameworks and insufficient accountability measures. Ongoing training is essential for all personnel handling and preserving evidence so that procedures and legal obligations are clearly understood.
Limited Storage Space and Strict Budgets
Many agencies have practical challenges, such as a lack of storage space and insufficient funds to acquire appropriate storage facilities for biological evidence. Budget cuts can also negatively impact training and result in obsolete or non-functional equipment. Plus, budget cuts limit resources to preserve investigative notes or manage other types of evidence effectively.
Technological Challenges
Evidence in digital formats poses unique preservation challenges. Data can become obsolete due to changing formats or hardware, and encrypted data may be inaccessible without the proper keys or permissions.
Cloud computing offers advantages such as easy storage and accessibility. However, it also raises jurisdiction and privacy concerns, requiring coordination with a state’s attorney general’s office in multi-state cases.
Best Practices for Evidence Preservation
Law enforcement agencies can follow several best practices to preserve evidence properly:
- Training and policy development – Regular staff training and documented standard operating procedures (SOPs) help clarify staff roles within evidence custody. Hands-on training through simulated crime scenes can also reinforce learning for not just detectives, but for anyone handling evidence.
- Technology and inventory systems – Barcodes, evidence management software, and live inventory tracking enhance evidence accuracy and transparency. Integration with national systems supports prosecutorial authority and compliance across jurisdictions.
- Interagency cooperation – Collaborative training, shared storage facilities, and uniform policies across competing jurisdictions increase reliability and improve evidence preservation. Collaborative agency work is critical across multiple jurisdictions or when federal and state partners must work together. For example, when a defendant commits crimes spanning multiple jurisdictions, cooperation ensures no evidence gaps emerge.
- Legislative and judicial oversight – Standard evidence-handling policies can also be enforced by legislatures, while courts can set rigorous standards for admissibility. Compliance is ensured by oversight bodies, accreditation processes, and continuous fulfillment, reducing risks of any claims that a government acted improperly.
- Community and victim advocacy group integration – The incorporation of community resources and victim advocacy groups strengthens transparency and improves public trust. These organizations push policies that focus on more long-term evidence retention, particular for sexual assault or homicide cases.
To maintain the integrity of the justice system, preserving evidence properly remains vital. Evidence supports the right to a fair trial, protects constitutional rights, and has the potential to right historical injustices. Evidence preservation is vital not just for crime solving and a conviction, but also for upholding the very pillars of justice and maintaining public confidence in our legal system.
Criminal Justice Degrees at AMU
For students who are interested in criminology, criminal investigation, digital forensics, and similar topics, American Military University (AMU) provides two bachelor’s degrees:
The courses in these bachelor’s programs feature a wide range of topics such as criminal law, the criminal legal process, and U.S. law enforcement. Other courses involve the U.S. legal system and criminal profiling. Also, both degree programs have several concentrations so that students can choose a concentration that best suits their academic and professional goals.
Want to learn more about these academic programs? Visit AMU’s criminal justice degree program page.
Note: Both degree programs are not designed to meet the educational requirements for professional licensure or certification in any country, state, province or other jurisdiction. These programs have not been approved by any state professional licensing body and do not lead to any state-issued professional licensure.
