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Golden State Killer, Part 1: Experts Explain Forensic DNA Evidence

This marks the first blog post I’ve written on a local event of national concern, the arrest of the Golden State Killer. The subject matter is sensitive for many involved, as such I aim to treat the topic with respect. There are significant issues related to the use of expert witnesses in a matter of this magnitude (DNA, ballistics, crime scene investigation and reconstruction, forensic psychology, forensic psychiatry, and more), so I have opted to address those issues with the input of appropriate experts.

In the week and a half since the arrest of suspected Golden State Killer, Joseph James DeAngelo, Jr., those of us in Northern California have been inundated with news reports about the crimes attributed to this prolific serial rapist and murderer who lived less than an hours drive from the Experts.com corporate office.

Prior to the arrest, I only knew bits and pieces about the history of the Golden State Killer (AKA: East Area Rapist, Original Night Stalker, Visalia Ransacker). Little did I know that several of the attacks (rapes) happened in my hometown of Stockton, California, putting the city and San Joaquin County on edge in late 1977 to early 1978. The Stockton attacks took place before and after many similar attacks in neighboring Sacramento and Stanislaus Counties, so the entire Central Valley was nervous. Attacks in Stockton apparently resulted in my parents installing their first alarm system.

How did the police locate the suspect?

The Sacramento Bee, has published many articles on the subject since the story first broke at the end of April. They have done some incredibly thorough reporting and I have used their reporting as a guide to writing this article.

It seems investigators had DNA samples from rape victims for decades. The only problem was the DNA samples did not match any of the DNA in criminal databases maintained by law enforcement. This means the suspect had never been caught for any crime and thus his DNA had never been submitted to one of the criminal DNA databases.

Following up on the Golden State Killer cold case, investigators decided to use an open-source genealogy database called GEDmatch. This is similar to the paid services of Ancestry.com and 23andMe, only it is mostly free to the public. Once the DNA profile was uploaded, investigators discovered a pool of potential candidates or relatives with similar genes. From there, they were able to eliminate suspects due to age, sex, location, etc. Eventually, they filtered down to the only suspect who made sense: Joseph James DeAngelo, Jr. After surveilling the suspect, DNA samples were obtained from items he discarded. Investigators then compared those samples to the Golden State Killer DNA. After a several attempts, detectives discovered a solid match and arrested DeAngelo.

DNA evidence is complicated, so I reached out to several of our members to discuss the DNA evidence involved in this case. Two Experts.com members responded to my questions for the first portion of this series on forensics.


Suzanna Ryan – Forensic Serology & DNA Expert Witness:

Blog readers may remember a really fun and informative live interview I did with forensic serology and DNA expert, Suzanna Ryan. She provided live and recorded viewers with some fascinating information on forensic DNA analysis. Naturally,  I reached out to her for input on the Golden State Killer case.

It is important for readers to remember we are only commenting on publicly available information from news reports. As in previous articles, I have outlined my questions and Ms. Ryan’s answers below. To learn more about her practice, please visit her website: ryanforensicdna.com.


Nick: What are the types/forms of DNA evidence law enforcement is likely to have retrieved from Golden State Killer crime scenes?

Ms. Ryan: I am not sure I know enough about the cases to say for sure, but it sounds like at least some of the cases involved rape, so it is likely that in the mid-70s and mid-80s the perpetrator was not being very careful about not leaving his DNA – in the form of semen – behind.  There is a lot of DNA in semen and it is likely that a clean, single source profile was able to be obtained from those type of samples.  That is probably how the cases were linked to each other as well.

Nick: Is there a type/form of DNA that is more compelling or stronger evidence than other types?

Ms. Ryan: Body fluid DNA is typically,  in my view, more compelling evidence because it is more difficult to argue that the DNA arrived there inadvertently through some sort of secondary transfer. This is especially true if the perpetrator and victim did not know each other and there would be no reason for the perpetrator’s blood to be in the victim’s home, for example, or his semen in her body.

Nick: According to news reports, the last known Golden State Killer crime took place in 1986. Does DNA evidence degrade over time?

Ms. Ryan: DNA evidence can degrade over time, but if it is properly stored (i.e. in paper, not plastic and in a temperature controlled environment like an indoor property room – NOT in an attic, garage, or non-temperature controlled shed of some sort) then DNA profiles can be obtained many years later.  Heat, humidity, and moisture are all very detrimental to DNA evidence.

Nick: How does law enforcement maintain DNA evidence over decades?

Ms. Ryan: Once a profile is obtained from an item of evidence, that profile is stored electronically.  However, evidence that has not been tested or is part of an unsolved case will be stored in police evidence rooms, categorized by case number.  It should remain sealed and in paper bags, manila envelopes, or boxes.

Nick: From public reports, we know law enforcement has found a DNA match. What type of evidence will the prosecution have to present at trial to prove the DNA belongs to the suspect?

Ms. Ryan: I don’t know the answer to this.  Likely semen DNA evidence.  Possibly blood or even touch DNA at some of the crime scenes if re-testing or additional testing of evidence is conducted.

Nick: Are there arguments the defense can use to claim the DNA is not from the suspect?

Ms. Ryan: I’m sure there are.  No way to answer this unless the case files are reviewed. However, when you have multiple cases linked together combined with a database-type match like this (in other words, he wasn’t a suspect before but was found through searching one type of database or another) it becomes more difficult to argue that there is some sort of laboratory error or cross-contamination in the lab.

Nick: Are there items you think the public should know about DNA evidence that I have not covered in the above questions?

Ms. Ryan: You didn’t really ask about the whole forensic genealogy aspect to this.  That the investigators used a private lab (not sure who, but I suspect Parabon) to do the same type of DNA typing (SNPs) that ancestry DNA labs do on an evidence sample from the case.  That profile was then uploaded to one (and frankly, probably more than one) DNA sharing site to search for a possible relative.  There is also some news about some other guy in a nursing home being a suspect, etc.  There was apparently similarities in his DNA profile to the GSK, so they got a warrant for his DNA, did a comparison, and found he was excluded.  That doesn’t mean they “got the wrong guy”, etc. that is being played up in the media.  It means that he was a possible suspect because of his DNA, but he was also cleared, because of his DNA.  The police probably followed around and surreptitiously collected DNA from a LOT of guys before they found DeAngelo.  That’s kind of how this type of search goes.  It is similar to, but not the same as familial DNA searching.  That is something the public and media don’t understand.  Familial searching uses the CODIS database to look for near matches.  It works because 46% of offenders have a brother or father who is also incarcerated.

George Schiro – DNA Technical Leader & Forensic Science Expert Witness:

George Schiro is the Lab Director of Scales Biological Laboratory in Brandon, Mississippi. His duties include incorporating DNA Advisory Board (DAB) standards, accountability for the technical operations of the lab, conducting DNA analysis in casework, DNA research, forensic science training, and crime scene investigation. I posed the same questions to Mr. Schiro that were presented to Ms. Ryan. You can learn more about him by visiting his website: forensicscienceresources.com.



Mr. Schiro: I know that seminal fluid was collected from at least some the crime scenes, and the GSK may have injured himself at some point during one of the many attacks and left blood behind at one or more of the scenes. These are the two most likely sources of DNA found at the scenes. From this biological evidence, they would have done autosomal short tandem repeat (STR) DNA testing on the stains. This generated DNA profiles that the crime labs were able to upload to the Combined DNA Index System (CODIS) DNA database. This was how they originally connected the East Area Rapist (EAR) with the Original Night Stalker (ONS) and knew they were committed by a single individual. The profile in CODIS did not hit on any identifiable individual in the database. A familial search was also conducted at the state level (SDIS) to look for any potential relatives in the CODIS database. No potential relatives were found in the database. They may have also conducted Y chromosome STR (Y STR) DNA analysis on the samples in an effort to find any potential relatives. It is my understanding that they found a rare genetic marker in one of the samples and used a publicly available genealogy database to trace the marker from family members in the database back to Joseph James DeAngelo. They then used a discarded item from Mr. DeAngelo to compare his autosomal STR profile to that of the GSK and they matched the profiles.


Mr. Schiro: Autosomal STR DNA analysis is currently the strongest type of DNA evidence because it remains consistent throughout a person’s life; with a few, rare exceptions, the DNA is the same throughout the individual’s body; the results are highly reproducible; the methods are standardized; and this type of DNA is highly individualizing. Theoretically, enough autosomal STRs are used to distinguish everyone on the planet with the exception of identical twins. Eventually, next generation sequencing (NGS) will probably supplement, and, perhaps, eventually replace autosomal STR DNA analysis as the crime lab tool of choice.


Mr. Schiro: DNA will degrade over time if it is exposed to heat, humidity, or ultraviolet (UV) light; however, if it is air-dried when collected and stored at room temperature under climate controlled conditions to reduce humidity, then it can last for decades, perhaps centuries. DNA has been obtained from mummies, so it is a very stable molecule under the right conditions.

Nick: How does law enforcement maintain DNA evidence over decades?

Mr. Schiro: Law enforcement maintains DNA evidence by air drying it upon collection, then keeping it stored in an air-conditioned, secure location where the temperature and humidity can be regulated.


Mr. Schiro: They will have to show that the evidence originated from the crime scenes. They can do this by introducing crime scene photographs, videos, investigator notes, and testimony of the people who collected the evidence. They will then have to show that the evidence was stored and preserved properly; and the evidence chain of custody was maintained. They will have to show that the items were tested using validated techniques and that a report was produced. The report can be introduced at trial and the DNA analyst can testify to the tests and results. They will then have to show that a sample was collected from Joseph James DeAngelo and all of the same steps were taken with his sample. Finally, they will have to show how the DNA samples from the crime scenes match the DNA sample from Mr. DeAngelo.


Mr. Schiro: In a cold case DNA hit, such as this one, and because the DNA is from seminal fluid and, possibly, blood, it will be very difficult to argue that Mr. DeAngelo is not the source of the DNA. Arguments of potential sample switches, contamination, and secondary transfer of DNA will not apply to this kind of case. About the only avenues left to the defense are to challenge the legality of the evidence collection and the legality of obtaining Mr. DeAngelo’s reference sample. They can also argue that Mr. DeAngelo has an identical twin who is the actual killer/rapist, but this might not be a realistic defense.


Mr. Schiro: No, I think you covered it pretty well. Thanks.

How about some further input:

My friend and colleague, Tamara McCormic of LegalForms.Today, is a legal advocate based in Orange County, California. Tamara works for the Orange County Public Defender in their exoneration unit. She is very knowledgeable about the use of DNA evidence in violent crimes. As several of the alleged murders took place in Orange County, this matter hits close to home for her as well.

I’ve asked Tamara to share her thoughts on the Golden State Killer matter based on her experience in homicide exoneration. A link to her blog post will be made available shortly.


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