Agencies should adopt policies to ensure that all evidence is collected, analyzed, and retained in a manner that is impartial, consistent, and thorough and that is designed to minimize any contamination or alteration of information.
a. Accuracy. In order to promote the goals of public safety and policing articulated in these Principles, agencies should seek to ensure the successful apprehension and conviction of the guilty, while clearing those who are not. Evidence collection is critical to this task. It facilitates drawing the most accurate conclusions about the underlying factual questions raised in criminal investigations. Agencies should strengthen the quality of evidence collection, and of any factual conclusions, through sound policy, officer training, and supervision.
b. Impartiality. Officers conducting investigations are charged with discovering the truth. They should maintain the appearance and the practice of impartiality in their investigations. They should not presume guilt or innocence but instead should remain neutral and objective. In particular, they must be at least as concerned with ruling out innocent suspects as with identifying criminal wrongdoers.
c. Consistency. These Principles govern not only officers working for policing agencies, but any officials who investigate crimes, or who collect, analyze, or retain evidence in criminal investigations. Such persons are referred to in these Principles as “investigators.” Crime-scene-evidence collectors include not only uniformed officers but also other individuals such as arson investigators, crime-scene investigators, coroners, criminalists, detectives, hospital personnel, forensic scientists, medical examiners, and photographers. After evidence is collected, other individuals play a role in analyzing and retaining the evidence, including staff at crime laboratories. The nature and process of crime-scene investigation varies dramatically across and within jurisdictions. There is the potential for inconsistent policies and procedures, miscommunications, errors, and biases. These Principles call for sound and uniform procedures and rules upon which all investigators should rely. Chapter 9 provides specific guidance for the collection of forensic evidence, which is defined in that Chapter as scientific or technical evidence for use in litigation. High quality evidence collection practices are essential, however, whether or not the evidence is considered as forensic evidence. That is the goal of this Chapter.
d. Thoroughness. Investigators should develop conclusions about an investigation only after assessing alternatives carefully, and always in recognition that any conclusions are contingent and subject to modification in light of new evidence. Investigators effectively are scientists and should act as such. Evidence collection and evaluation, including decisions not to pursue leads, should be documented in a manner that facilitates re-evaluation. Officers should pursue leads diligently as to evidence that might shed light on a suspect’s guilt or innocence, but also across a range of other types of evidence, including evidence that can help to assess the credibility of witnesses, the manner in which the crime took place, and the identities of possible alternative suspects, as well as evidence that might shed light on an alibi. Investigations should not be closed prematurely, for example, upon a suspect’s arrest or confession. Instead, all evidence should be evaluated carefully, including, when possible, by soliciting the views of officers who are independent from the investigation and can offer outside suggestions.
e. Rights. Evidence collection should be conducted in a manner that is compatible with individual rights. Individual-rights protections are discussed separately in Chapters 2 and 3 on search and seizure, Chapter 10 on eyewitness-identification procedures, and Chapter 11 on police questioning.
f. Dignity and respect. Similarly, consistent with the discussions of fairness and procedural justice throughout these Principles, efforts to obtain reliable evidence should be conducted in a manner that avoids unnecessary harm to persons or property and treats people with dignity and respect.
1. Accuracy of evidence gathering. The overarching goal of police evidence gathering is to ensure that accurate evidence is collected to identify culprits and to clear innocent suspects. Toward that goal, agencies have long adopted techniques designed to collect evidence in as reliable a manner as possible, to safeguard the integrity of evidence so that it can later be relied upon, and to disclose that evidence so that legal actors are well informed. New technologies and scientific research have made available improved techniques for evidence collection, but they have also introduced new challenges. For example, DNA technology has made it possible to collect highly probative evidence concerning the identity of culprits, but it also requires special expertise in evidence collection to prevent contamination.
In Chapter 9, we further detail the need for repeatable, reproducible, and accurate work with regard to forensic evidence of a scientific or technical nature. Although scientific standards need not necessarily apply to evidence that is not of a scientific or technical nature, agencies nevertheless should aim to collect all evidence in a manner that assures it is not contaminated, altered, or less than accurate. In Chapter 9, describing accuracy in the context of forensic tests, we state that “accurate” means that, with known probabilities, an examiner obtains correct results both (1) for samples from the same source (true positives) and (2) for samples from different sources (true negatives). The same concept of accuracy applies to any form of evidence collection and analysis, where one is concerned not just with matching samples to sources, but rather a broader range of conclusions that may either result in false positive, false negative, or entirely inconclusive determinations. We also note that many types of evidence may have scientific or technical aspects to them, for which the principles set out in this Chapter and in other Chapters also may apply. For example, eyewitness accounts implicate a large body of scientific research and eyewitness memory is tested using lineup procedures, as discussed in Chapter 10.
2. Constitutional rulings. Constitutional law regulates certain egregious mishandling of crime-scene-evidence gathering. For example, constitutional criminal procedure bars the failure to disclose exculpatory evidence and the outright fabrication of evidence by law enforcement. The U.S. Supreme Court has long held that it is a violation of the Due Process Clause to fabricate evidence or knowingly use perjured testimony. Napue v. Illinois, 360 U.S. 264 (1959); Mooney v. Holohan, 294 U.S. 103 (1935). In addition, officers and prosecutors must supply “any favorable evidence known to others acting on the government’s behalf in the case, including the police.” Kyles v. Whitley, 514 U.S. 419, 437 (1995); Brady v. Maryland, 373 U.S. 83 (1963). As a result, agencies and prosecutors are treated as part of a joint enterprise and are held accountable jointly for violations of the Due Process Clause. Lower courts routinely have entertained civil damages suits by individuals alleging that law enforcement withheld exculpatory evidence. See, e.g., Gregory v. City of Louisville, 444 F.3d 725, 739 (6th Cir. 2005); Pierce v. Gilchrist, 359 F.3d 1279, 1300 (10th Cir. 2004); Atkins v. County of Riverside, 151 Fed. Appx. 501, 505-506 (9th Cir. 2005); Hunt v. McDade, 2000 WL 219755, at *4 (4th Cir. Feb. 25, 2000); In re Brown, No. 19-0877, 2020 WL 7413728 (Tex. Dec. 18, 2020). These disclosure obligations encompass providing adverse information concerning police officers (including information concerning internal investigations) who may be witnesses in criminal litigation.
In addition, the U.S. Supreme Court has found that highly unreliable evidence-collection practices, for example, in the area of eyewitness identifications, may violate the Due Process Clause. The Court’s ruling in Manson v. Brathwaite, 432 U.S. 98, 114 (1977), set out a due-process test using a series of factors that can permit eyewitness identifications in which police used suggestive procedures to be admitted into evidence. In 2012, the Court did rule that state action must occur to trigger this due-process inquiry, but the justices did not revisit the Manson v. Brathwaite test itself. Perry v. New Hampshire, 565 U.S. 228 (2012). Subsequent scientific research, discussed in Chapter 10, has shed light on the shortcomings of this due-process test and has provided a set of best practices that can improve the accuracy of eyewitness evidence. See also Nat’l Research Council of the Nat’l Acads., Identifying the Culprit 104 (2014).
The U.S. Supreme Court has not clearly recognized a freestanding right of a convict to claim innocence, which might allow a convict to challenge wholly unreliable evidence gathering or analysis by law enforcement. Nevertheless, innocence claims are litigated in state and federal court, providing an added incentive for police agencies to ensure that evidence of innocence is uncovered before and not after a conviction. See Herrera v. Collins, 506 U.S. 390, 401, 403–404 (1993) (declining to recognize, except hypothetically, freestanding constitutional actual-innocence claim in capital case); House v. Bell, 547 U.S. 518 (2006) (continuing to recognize actual-innocence claim hypothetically); Osborne v. District Attorney’s Office, 557 U.S. 52, 69 (2009) (recognizing actual-innocence claim hypothetically in noncapital case); see also Brandon L. Garrett, Claiming Innocence, 92 Minn. L. Rev. 1630, 1670-1675 (2008) (describing state-law remedies for post-conviction claims relying on newly discovered evidence of innocence).
3. Police procedures. Traditional police manuals contained detailed information concerning the constitutional law of search and seizure but often lacked the same attention to rules designed to promote reliability in investigations. A 1990s study of such manuals in six states found that “none of the training materials addresses the importance of investigating, recording, or reporting exculpatory facts to avoid punishment of a possibly innocent arrestee [, but rather they] reflect a psychological set in which the arrestee’s guilt is presumed, and the only use of notes and reports in the criminal process is to ensure conviction.” Stanley Z. Fisher, “Just the Facts, Ma’am”: Lying and the Omission of Exculpatory Evidence in Police Reports, 28 New Eng. L. Rev. 1, 18 (1993). In practice, police may not have policies or practices that strongly encourage documentation of exculpatory evidence. See id. at 30 (concluding that in the arrest-report process, “[m]ost police probably do not generally report exculpatory evidence.”). Moreover, other actors who participate in investigations, such as crime-scene units and laboratories, often have inconsistent policies and practices, or lack them altogether. The lack of consistent and uniform standards and oversight can result in errors ranging from fraud to honest mistakes made because of haste, inexperience, or lack of a scientific background. Nat’l Research Council of the Nat’l Acads., Strengthening Forensic Science in the United States: A Path Forward 56-57 (2009).
Over the past two decades, however, the situation has improved significantly. Modern efforts to create comprehensive policies on the gathering, documentation, and disclosure of evidence are altering a traditional lack of standards or regulation concerning the evidence-gathering function in police investigations. Detailed policies and procedures now reflect the common concern that appropriate evidence-collection procedures ensure accuracy, and that the integrity of evidence be safeguarded. See, e.g., Forensic Servs. Div., Oregon State Police, Physical Evidence Manual, http://www.crime-scene-investigator.net/Phys_Evid_Manual_OR.pdf (“It is important that evidence be collected, handled, and stored in a way that will ensure integrity.”). One impetus for change has been the advent of modern DNA testing, which since 1989 has led to hundreds of exonerations (i.e., official decisions to reverse a conviction based on new evidence of innocence). See Brandon L. Garrett, Convicting the Innocent: Where Criminal Prosecutions Go Wrong 168-170 (2011). There is no comparable body of DNA exonerations elsewhere in the world. Over the past several decades, there also have been thousands of additional exonerations that did not rely on DNA testing. See National Registry of Exonerations, https://www.law.umich.edu/special/exoneration/Pages/about.aspx. These exonerations have underscored how eyewitnesses can misidentify innocent suspects, forensic evidence can be unreliable and flawed, confessions can be false, and informants can lie on the stand. They also have resulted in scholarship studying wrongful convictions and new legislation regulating subjects including eyewitness-identification procedures and videotaping of interrogations. States also have relaxed rules of finality to make it easier to obtain access to evidence like DNA testing post-conviction, and to reopen convictions based on new evidence of innocence. Garrett, Claiming Innocence, supra.
Today, a host of police policies and guidelines reflect the growing effort to ensure accurate and impartial evidence collection. A New Jersey field manual puts it well: “The importance of physical evidence in a case cannot be underestimated. The credibility and integrity of evidence are predicated directly upon the proper handling of the evidence from its initial observance through presentation in court.” Investigations Branch, Off. of Forensic Servs., New Jersey State Police, Evidence Field Manual 4 (Jan. 2014), http://www.njsp.org/divorg/invest/pdf/013014_evidencefieldmanual.pdf. The International Association of Chiefs of Police (IACP) has taken an active role in promoting the consideration of ways to improve the “accuracy and thoroughness” of police investigations. Int’l Ass’n of Chiefs of Police, National Summit on Wrongful Convictions: Building a Systemic Approach to Prevent Wrongful Convictions 10 (Aug. 2013). The IACP has recommended improved training and supervision in order to improve the quality of investigations and evidence gathering. Id. at 11. Additional policies are directed toward accurate and effective crime-scene investigations as well, particularly given the rise in the ability to conduct forensic analysis of crime-scene evidence. See Technical Working Group on Crime Scene Investigation, Crime Scene Investigation: A Guide for Law Enforcement 17-32 (2000). Such work may be conducted by officers working for police departments, in conjunction with crime-lab analysts, technicians, and others. See Joseph Peterson & Ira Sommers, The Role and Impact of Forensic Evidence in the Criminal Justice Process 22 (2010).
In its Code of Police Ethics, the IACP makes clear the critical imperative here: “The law enforcement officer shall be concerned equally in the prosecution of the wrong-doer and the defense of the innocent. He shall ascertain what constitutes evidence and shall present such evidence impartially and without malice.” Int’l Ass’n of Chiefs of Police, Law Enforcement Code of Ethics, https://www.theiacp.org/resources/law-enforcement-code-of-ethics. This imperative is reflected in state law and in the policies or codes of ethics of an increasing number of policing agencies. See, e.g., id., adopted 37 Tex. Admin. Code § 1.113; Austin Police Dep’t Pol’y Manual, Article 10 (2020), https://www.austintexas.gov/sites/default/files/files/Police/General_Orders.pdf; Narragansett Police Dep’t, Policy 100.3, Article 10 (2019), https://www.powerdms.com/public/NSETTPD/tree/documents/371277; Federal Heights Police Dept., Directive 10.2 (2004), at http://www.fedheights.org/vertical/Sites/%7B30BDEC4F-3AAB-430C-A5CC-E2BE8097AC8C%7D/uploads/code_of_ethics_10_2.pdf. These Principles require that the aspiration of the Code of Ethics be reflected in detailed policy, training, and supervision concerning police evidence collection.