Nate Shoemaker

I. Introduction

DNA evidence is a powerful weapon in investigators’ and prosecutors’ arsenals in proving the identity of an individual suspected of committing a crime. It is powerful enough that the average true crime podcast or streaming enjoyer likely believes DNA evidence to be the final brick in the wall in proof beyond a reasonable doubt. DNA is not without flaws,[1] but is nonetheless very probative evidence.

There are two main kinds of DNA testing used in criminal investigations today: short tandem repeat (“STR”) and single-nucleotide polymorphism profiling (“SNP”). SNP profiling was recently at the forefront of legal proceedings against the man accused of killing four University of Idaho college students in November of 2022.[2] This is because the constitutionality of SNP profiling has been questioned with greater frequency, especially when it is used in a practice called investigative genetic genealogy (“IGG”).

II. Overview of STR and SNP Profiling[3]

DNA is highly complex, and DNA forensic testing even more so. STR analysis has been described as[4]

a cornerstone of modern forensic DNA analysis. This ingenious approach capitalizes on the short…repetitive sequences found scattered throughout the human genome, at specific locations of chromosomes…These sequences consist of a few nucleotide base pairs repeated in tandem, creating intricate patterns that vary from one individual to the next, except in cases of identical twins. By leveraging these variations, forensic experts can construct DNA profiles that are as distinctive as genetic fingerprints, enabling them to identify individuals with unprecedented accuracy.

STR analysis uses about twenty markers to create these distinctive profiles and is, by and large, the dominant form of DNA profiling in criminal investigation.[5] SNP analysis, on the other hand, “focuses on tiny variations at individual points in the DNA sequence. These variations…provide a detailed genetic roadmap. Unlike STRs, SNP profiles examine hundreds or even millions of data points.”[6] From these data points, SNP analysis can reveal “information about disease carrier status, predictive wellness, and cosmetic conditions, relying on hundreds of thousands of SNPs rather than the 13-20 STRs in the typical forensic profile. Compared to STRs, SNPs pack a lot more information, some of it potentially quite precious – in a profile.”[7]

III. Overview of IGG

As indicated above, STR profiling creates DNA profiles that are just like fingerprints. Like fingerprints, STR profiles must be directly compared to a known sample; this means that a suspect’s DNA profile taken from a crime scene would need to be contained in a database or compared against a DNA sample from someone already in custody on suspicion of the crime.[8] If there is no database match or there is no individual in custody to compare to, an STR profile is nearly useless. SNP profiling, on the other hand, can lead to identification of a suspect without a comparison sample through the practice of IGG. IGG involves the development of an SNP profile that is then “uploaded into a public, searchable genetic database,” such as MyFamilyTree DNA, GEDmatch, or 23andMe.[9] From there, investigators begin to build a family tree from the suspect SNP profile by comparing the similarities to other public profiles on the website; the closer the similarity between the suspect profile and the public profile, the closer the relation.[10] Once a suspect is identified through family tree analysis, investigators must obtain a DNA sample from that individual in order to conduct more conventional STR comparison.[11] In fact, the Department of Justice’s interim policy on IGG requires this step before an arrest can occur; IGG matches are not enough to warrant an arrest of a suspect on their own.[12] Thus, if the suspect does not consent to providing a DNA sample while out of custody, law enforcement will likely need to be creative in obtaining a sample.[13]

IV. IGG in Notable Cases

IGG requires a lot of time and resources to identify a possible suspect, including development of the initial SNP profile, extensive family tree building, and old-school policework in obtaining a sample from the identified suspect for STR comparison.[14] As such, use of IGG is typically limited to cold cases with little other investigative options.[15] The most prominent example of IGG is the Golden State Killer case, who terrorized California with a string of homicides and sexual assaults between 1974 and 1986.[16] SNP profiling and IGG ultimately led to the capture of a suspect in 2018, thirty-two years after his final recorded act of violence.[17]  As noted in footnote 13, IGG techniques resulted in the identification and capture of two cold case suspects decades after their respective crimes in our own backyard.[18] In Westrom, a woman was found dead in her Minneapolis apartment in 1993, having been stabbed sixty-five times in her chest and abdomen.[19] Possible DNA from the crime scene was found, but it did not lead to apprehension of any suspects.[20] IGG investigation of the sample began in 2018, and a suspect was identified and confirmed via STR profiling that same year.[21]

In Carbo, a woman was found sexually assaulted and strangled to death in her bed in the city of Chisholm in 1986.[22] Despite multiple sources of suspect DNA found at the scene, a suspect was not identified, and the case went cold.[23] IGG investigation began in 2019, and a suspect was ultimately identified, confirmed via STR profiling, and indicted for the homicide in 2022.[24]

Bucking the cold case trend is the Idaho college student quadruple homicide, where the suspect was identified via IGG techniques despite the underlying crime occurring only in 2022.[25] At the crime scene, investigators found a knife sheath that appeared to match the knife wounds suffered by the victims and located a DNA sample on the button. When STR profiling was conducted, there were no matches in any databases. Investigators thus turned to IGG and identified the defendant, who was later confirmed by STR profiling from his known sample. The defendant’s counsel filed a motion to suppress the DNA evidence and identification obtained through IGG, but this motion was summarily denied by an Ada County, Idaho judge on February 19, 2025.[26]

V. Constitutional Challenges to IGG

Among the challenges filed by the defendant’s counsel were that the SNP profiling was a Fourth Amendment violation.[27] Namely, the defendant argued that the advanced nature of SNP profiling and the sensitive data that it can identify about a person are private and subject to constitutional protection.[28]

In order to prevail on a Fourth Amendment challenge, a criminal defendant has to demonstrate that they had both a subjective expectation of privacy in the thing searched, and that the defendant’s expectation of privacy is objectively reasonable and readily recognized by society.[29] In all cases identified above—Westrom, Carbo, and the Idaho murders—the respective courts held that the defendant had no subjective expectation of privacy in the DNA left behind at their crime scenes. This conclusion was reached on grounds that the DNA was abandoned by each respective perpetrator and that each respective perpetrator thus had no reasonable expectation of privacy in testing done on that DNA.

Despite the denial of each motion, there is some momentum in recognizing a privacy interest in the detailed and “non-fingerprint” information revealed by SNP profiling, which is headlined by Supreme Court of Minnesota Justice Karl Procaccini’s concurrence in Carbo.[30] The Supreme Court of the United States has held that “if the seizure of an object does not violate an expectation of privacy, the subsequent testing of the object to reveal further information may yet implicate additional privacy interests.”[31] In this vein, although Skinner was not cited by Justice Procaccini, Justice Procaccini stated that the Carbo defendant retained an expectation of privacy in the genetic information revealed by SNP testing—even if the substance itself did not have such an expectation.[32]As noted, in the Idaho murders defendant’s case, defense counsel’s argument on the motion to suppress was based entirely on Justice Procaccini’s concurrence. Responding to this argument on an objectively reasonable expectation of privacy in SNP profile information, the Idaho court stated the use of such information is critical in determining whether a constitutional violation occurred.[33] Although sensitive medical information was available to see from the SNP profile, such information was not used; the SNP profile was only used for identification.[34] Because the SNP profile was limited to identification, there was no objectively reasonable expectation of privacy.[35]

VI. Conclusion

As the Idaho college murder defendant’s case proceeds to trial, all DNA evidence will be admissible. If there is an expectation of privacy to be retained in SNP profiling, it is likely far from being judicially recognized if IGG investigation maintains the status quo. On the other hand, if an IGG investigation starts to wade into more sensitive medical information beyond what is necessary for identification, courts will be much more likely to step in. But until then, IGG is likely out of reach of the Fourth Amendment.

 

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[1] DNA is such powerful evidence that it can be subject to false confidence. This was the case with Amanda Knox, an American student who was accused of killing her roommate in Italy. In a highly publicized legal saga, Knox was convicted, acquitted, re-convicted, and acquitted again based on minuscule amounts of her DNA found on a kitchen knife and her roommate’s bra. Colleen Barry & Frances D’Emilio, Italian Court Throws Out Knox Conviction Once and For All, AP News (Mar. 27, 2015), https://apnews.com/general-news-018d6a0ba12845aba0dfa91adbef968e.

[2] Stephen J. Beard, Jennifer Borresen & Dinah Voyles Pulver, Timeline of Idaho killings: When and Where the Victims Were Targeted, USA Today (Jan. 19, 2023), https://www.usatoday.com/in-depth/graphics/2023/01/14/idaho-college-murders-timeline/11020578002/.

[3] I cannot stress enough that this is a ten-hundred-thousand-foot overview of either kind of DNA testing; these descriptions are very reductive of the science and processes used to develop the respective types of profiles, but serve to illustrate how they each function with respect to the Fourth Amendment in a digestible way.

[4] A Comprehensive Exploration of STR Analysis, Simplyforensic (Aug. 17,2024), https://simplyforensic.com/a-comprehensive-exploration-of-str-analysis/.

[5] STR vs. SNP: Why Genetic Genealogy Relies on SNP Testing, BODE Technology, https://bodetech.com/str-vs-snp-why-genetic-genealogy-relies-on-snp-testing/ (last visited Mar. 18, 2025).

[6] Id.

[7] Erin Murphy, Law and Policy Oversight of Familial Searches in Recreational Genealogy Databases, 292 Forensic Sci. Int’l e5 (2018).

[8] BODE Technology, supra note 5.

[9] Leighton D’Antoni, Investigative Genetic Genealogy (IGG): A Guide for Prosecutors, The Texas Prosecutor: Texas Dist. & Cnty. Attorneys Ass’n, https://www.tdcaa.com/journal/investigative-genetic-genealogy-igg-a-guide-for-prosecutors/ (last visited Mar. 18, 2025). These websites give users the option of making their profiles available to criminal investigators for comparison. Law enforcement cannot access profiles for IGG if the user does not consent.

[10] Id.

[11] Id.

[12] United States Dep’t of Justice, Interim Policy: Forensic Genetic Genealogical DNA Analysis and Searching (2019). As noted by D’Antoni, this 2019 “interim” policy does not appear to have been overridden or replaced by another policy since it was made effective.

[13] Compare State v. Westrom, 6 N.W.3d 145 (Minn. 2024) (explaining that police covertly followed an IGG-identified suspect at a hockey game and pulled his used napkin from a trash can, DNA from which was an STR match to DNA from a 1993 crime scene) with State v. Carbo, 6 N.W.3d 114 (Minn. 2024) (explaining that police approached an IGG-identified suspect in public and obtained a DNA sample by simply asking without explaining why, which was an STR match to DNA from a 1986 crime scene).

[14] D’Antoni, supra note 9.

[15] Cholistani, et. al., Forensic Genealogy: A Powerful Investigation Tool for Resolving the Cold Cases, 12 J. Forensic Sci. & Criminology 3 (2024).

[16] Thomas Fuller & Christine Hauser, Search for ‘Golden State Killer’ Leads to Arrest of Ex-Cop, N.Y. Times (Apr. 25, 2018), https://www.nytimes.com/2018/04/25/us/golden-state-killer-serial.html?action=click&module=RelatedCoverage&pgtype=Article&region=Footer.

[17] Ray Wickenheiser, Forensic Genealogy, Bioethics and the Golden State Killer Case, 1 Forensic Sci. Int’l 114 (2019).

[18] State v. Westrom and State v. Carbo, supra note 13.

[19] State v. Westrom, 6 N.W.3d 145, 151 (Minn. 2024).

[20] Id.

[21] See id.

[22] State v. Carbo, 6 N.W.3d 114, 119 (Minn. 2024).

[23] Id.

[24] See id.

[25] Erik Ortiz, Idaho College Student Killings: A Summary and Timeline, NBC News (published Dec. 31, 2022; last updated Mar. 13, 2025), https://www.nbcnews.com/news/us-news/idaho-college-student-killings-summary-timeline-rcna63818.

[26] See Order on Defendant’s Motion to Suppress Re: Genetic Information (Feb. 19, 2025), State of Idaho v. Bryan C. Kohberger, Ada County Case No. CR01-24-31665.

[27] See id. The defendant also challenged the constitutionality of a trash pull that provided DNA samples from his parents that helped build the case against him, but this issue is not relevant to the larger issue of SNP profiling. The defendant also challenged the use of the third-party genealogy websites, but this claim was rejected for lack of standing.

[28] See id.

[29] Katz v. United States, 389 U.S. 347, 360 (1967) (Harlan, J., concurring).

[30] State v. Carbo, 6 N.W.3d 114, 119 (Minn. 2024) (Procaccini, J., concurring). In fact, the order from the Ada County judge notes that Justice Procaccini’s concurrence was the sole support for the Idaho murder defendant’s motion to suppress. Although Justice Procaccini disagreed with the Carbo majority’s decision finding no Fourth Amendment violation in SNP profiling, Justice Procaccini agreed with denying the motion to suppress, instead applying the good-faith-reliance-on-binding-precedent doctrine to forgive the constitutional violation.

[31] Skinner v. Ry. Lab. Execs.’ Ass’n, 489 U.S. 602, 616 (1989).

[32] State v. Carbo, 6 N.W.3d 114, 119 (Minn. 2024).

[33] Order on Defendant’s Motion to Suppress, supra note 26, at 15.

[34] Id.

[35] Id.