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Sexual Assault Cases: Exploring the Importance of Non-DNA Forensic Evidence

Investigating and prosecuting sexual assault crimes is much more complicated than simply performing DNA testing.
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National Institute of Justice Journal
Date Published
November 9, 2017
collage of different types of forensic evidence
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After an evening of hanging out with friends, a 20-year-old woman decided to get a ride home with her ex-boyfriend. They had broken up several months before but had remained friendly with each other. During the drive, the young man started talking about how they should get back together because he missed the relationship they once had. However, she was not interested and wanted to remain friends.

Only blocks from her home, the man stopped the car and suddenly turned violent. Grabbing her neck, he began to force himself on her. As he ripped off her clothes, she managed to grab a small pocket knife from her purse, but he took the knife from her and broke it. He then dragged her by the legs out of the car and raped her in a nearby wooded area. Afterwards, he stole her credit cards and drove away. The woman managed to walk to her home and called the police to report the rape. She was brought to the hospital, where she was interviewed, photographed, and examined for several hours. Evidence was collected for a sexual assault kit (SAK).

To many, this would seem like a clear-cut case of sexual assault. However, a case is not determined by what one believes, but rather by what the investigation shows and what can be proved.

When the police spoke with the young man, he described a very different series of events. He claimed that he and his ex-girlfriend had a fun evening at the party and that she came on to him and wanted to get back together. He admitted to taking some drugs that evening and thought he blacked out briefly. When he woke up, he claimed that his ex-girlfriend was on top of him and they had consensual rough sex. He said their clothes were everywhere, and he must have accidently taken her credit cards when he gathered his things. He denied ever handling a knife.

So what really happened that evening? In a case like this of “he said, she said,” investigators look to the forensic evidence to help them piece together what may have actually happened. The investigation and exploration of the totality of the evidence collected are critical to unravelling the allegations and discerning whether charges can be filed.

DNA Is Not Always Probative or Present

DNA analysis has had an unprecedented impact on the criminal justice system. It has propelled investigations forward and made charging those alleged to have committed the crime easier. Science can now provide focus or direction to an investigation, help develop a case theory, and clear suspects or those wrongfully convicted with more certainty. As a result, the criminal justice community has become highly reliant on DNA analysis.

Many believe DNA analysis is the death knell for most criminal defendants, and juries and lawyers alike expect to see DNA evidence presented during a trial.[1] It has been reported that 72 percent of jurors anticipate seeing DNA in a sexual assault trial[2] and that juries are 33 times more likely to convict when presented with DNA evidence.[3] This raises serious concerns, as the need to provide probative DNA evidence in sexual assault cases has become increasingly more important if a prosecutor hopes to secure a conviction.

But what happens in a “he said, she said” case like the one described above, where the alleged suspect is not a stranger and, in fact, admitted to having sexual intercourse? In a random sampling of 602 reported rapes, 36.2 percent were among intimates or family members and 42.7 percent were friends or acquaintances.[4] In such cases where the suspect and victim are acquainted, consent is pivotal to determining whether a crime was committed, and the presence of the suspect’s DNA may not necessarily be informative.[5]

Or what about cases in which no DNA is found? Recently, a great deal of attention has been placed on testing SAKs, especially those collected years ago and never submitted to a forensic laboratory. Although testing SAKs for DNA is important, the reality is that DNA is sometimes not found or is not eligible for entry into the Combined DNA Index System (CODIS),[6] which is composed of databases maintained by the Federal Bureau of Investigation that allow DNA profiles to be compared to one another. For example, to be uploaded into CODIS, DNA profiles must meet certain quality standards, and sometimes the DNA is old, degraded, not in sufficient quantity, or otherwise unviable. Additionally, the sample may not be directly probative (such as in cases in which the person alleged to have committed the crime admitted having sexual contact).[7] Based on NIJ’s SAK-related projects (see exhibit 1), DNA profiles that were of sufficient quality for CODIS upload were obtained in only 38 percent of more than 7,000 SAKs submitted for DNA testing.[8] Also, performance metrics collected from NIJ’s Solving Cold Cases With DNA program showed that about 48 percent of the cases with tested biological evidence yielded any DNA profile.[9] NIJ’s results are consistent with other national findings. For example, a recent South Dakota SAK project that tested 504 kits yielded only 254 DNA profiles,[10] and half of the 3,542 kits tested in a Colorado project yielded DNA profiles.[11]

As new and emerging technologies advance across the criminal justice system and provide necessary links for locating and apprehending assailants, criminals continue to learn about forensic science techniques and have started to “get smart.” Many individuals who commit sex offenses, for example, now use gloves, masks, and condoms, and some even have a victim shower before they leave a scene — all in the hope of thwarting law enforcement’s ability to collect potential DNA evidence. Take, for example, the case of Colorado serial rapist Marc O’Leary, who committed several sexual assaults in multiple jurisdictions. He ordered women to shower and brush their teeth, and he took the bedding and clothing with him. However, he executed his crimes with the same modis operandi, leaving behind shoe prints, glove pattern evidence, and other non-DNA evidence that was used to connect him to the crime scenes.[12]

Exhibit 1. Data From NIJ SAK Programs


Number of SAKS Tested

Number of CODIS Entries

% of Profiles Eligible for CODIS Entry


CODIS Hits/ CODIS Entries (%)

CODIS Hits/Total Number of SAKS (%)

Los Angeles[*] 1,948 699 36% 347 50% 18%
Detroit[**] 1,595 785 49% 455 58% 29%
New Orleans[***] 1,008 256 25% 139 54% 14%
Houston[^] 491 213 43% 104 49% 21%
FBI (as of 1/5/2017)[^^] 1,584 808 51% 306 38% 19%
Marshall (as of 10/5/2016)[^^^] 588 149 25% 39 26% 7%
Total 7,214 2,910   1,390    
Average     38%   46% 18%


The Value of Non-DNA Evidence

In cases in which probative DNA evidence is not readily available or offers little or no meaning to the allegations being made, an accumulation of non-DNA forensic evidence can be what ultimately leads to a successful conviction.

A wealth of other forensic evidence may be invaluable in sexual assault investigations; some examples are trace evidence (e.g., hairs, fibers, glass, paint, or soil), toxicology, cellphone and digital forensics, and impression and pattern evidence (e.g., fingerprints, shoe prints, tire marks, and handwriting). One study showed that in addition to bodily fluids, fingerprints and hairs are the most common types of physical evidence collected and examined in sexual assault casework.[13] The Bureau of Justice Statistics also reported that from 2005 through 2010, individuals committing sexual assault were armed with a gun, knife, or other weapon in 11 percent of rape or sexual assault victimizations.[14] All of these types of non-DNA forensic evidence can be used to identify a suspect, associate a suspect with a victim, associate a suspect with a crime scene, and corroborate other evidence.

Based on the evidence identified and collected during the sexual assault investigation in our case example, the knife was tested and contained prints of both the victim and the suspect, even though the suspect denied handling the knife. The young woman had no alcohol in her system, and the medical exam showed she had cuts, bruises, and tears to her body and genitals — 33 documented injuries. Her clothing was torn and her pants zipper was broken. Toxicology results revealed that the suspect tested positive for methylenedioxy-methamphetamine (MDMA, commonly known as Ecstasy), a drug that is known to produce feelings of increased energy, pleasure, and distorted sensory and time perception. The investigation also revealed that the suspect sent the victim text messages in the days after the assault, even though he denied having any contact with her.

Therefore, when DNA is not available or not probative, other forensic evidence can help establish the facts. The most important take-away is that a case should be developed using the totality of the evidence rather than relying solely on DNA, which allows one to recreate an entire series of events; corroborate or refute testimony from the victim, suspect, or other witnesses; and ultimately include or exclude a potential suspect. Based on the accumulated evidence described in this case scenario, criminal charges would likely result,[15] as there is evidence showing injury and violence, corroboration of the victim’s account of the attack, and evidence that would disprove aspects of the suspect’s version of events.

Prosecutors also rely on scientific evidence to establish and prove their cases during prosecution. Research has shown that “… analysis of forensic evidence was associated with [increased] case referrals to prosecutors, [and] charges filed ….”[16] Evidence from a medical forensic exam can be key in the prosecution of a sexual assault case. A study conducted in 1999 found a relationship where the victims’ injuries were seen as one of the most significant predictors in the decision to prosecute in nonstranger cases.[17] The presence of injuries seemed to influence a prosecutor’s decision-making, as the documented appearance of violence makes it more difficult to assert consent.

The presence of forensic evidence also strengthens the likelihood of conviction at trial and has been associated with harsher sentences.[18] Prosecutors are better able to recreate events and illustrate a theory to a jury by connecting testimonial evidence and forensic evidence. More specific to sexual assault cases, research has indicated that cases with physical evidence were more likely to lead to arrest, be referred to the prosecutor, be charged, and result in conviction than cases without evidence.[19] Sometimes the availability or admission of forensic evidence, merely to show its existence, can be essential as well. Regardless of the context or support the evidence may offer to a case, juries continue to expect personally identifying forensic evidence; for example, Ric Ridgway, chief prosecutor for the 5th Judicial Circuit in Central Florida, received feedback from jurors after an acquittal asserting, “Well, there wasn’t any DNA or fingerprints.”[20]

The Need for Research and Innovative Non-DNA Forensic Methods

Countless types of forensic evidence are used to investigate and prosecute thousands of crimes annually. According to the Bureau of Justice Statistics, in 2014 the nation’s 409 crime labs received an estimated 3.8 million requests for forensic services. Only 9 percent of these requests were for forensic biology casework such as from a crime scene (which includes DNA testing), and 24 percent were for DNA analysis of reference samples collected from those convicted and arrested that were then added to the national database.[21] This means 67 percent of the total requests were solely for non-DNA forensic analysis.

Exhibits 2 and 3 illustrates the types of evidence analysis requested from publicly funded laboratories between 2009 and 2014. Clearly there is a demand for non-DNA forensic evidence. Toxicology testing, or the identification of drugs or other chemicals in the human body, was the third most requested of all forensic evidence, after forensic DNA sample testing. Controlled substance analysis of drugs or chemicals regulated by the government, such as cocaine, heroin, marijuana, or certain prescription drugs, was first.

Toxicological analysis has become critical in the prosecution of drug-facilitated sexual assault cases as another way to nullify an assertion of consent. To further advance science and ensure that evidence can be identified and used appropriately, NIJ supports and funds a diverse portfolio of forensic science research to develop highly discriminating, accurate, reliable, cost-effective, and rapid methods for the identification, analysis, and interpretation of physical evidence for criminal justice purposes.

See “Drug-Facilitated Sexual Assaults.”

Exhibit 2: Requests for Services Received and Completed by Publicly Funded Forensic Crime Labs, by Type of Request, 2009 and 2014
Type of request Number, 2009 Percent, 2009 Number, 2014 Percent, 2014
Controlled substances 1,358,000 34 1,265,000 33
Crime scene 188,000 5 171,000 5
Digital evidence 33,000 1 25,000 1
Firearms/toolmarks 147,000 4 154,000 4
Forensic biology casework 260,000 6 333,000 9
Forensic biology from samples taken from individuals convicted/arrested  1,053,000 26 908,000 24
Impressions 11,000 -- 7,000 --
Latent prints 270,000 7 295,000 8
Questioned documents 13,000 -- 9,000 --
Toxicology 629,000 16 566,000 15
Trace evidence 58,000 1 49,000 1
All requests 4,020,000 100% 3,783,000 100%
Exhibit 3: Requests for Services Completed by Publicly Funded Forensic Crime Labs, by Type of Request, 2009 and 2014
Type of request 2009 Number 2009 Percent 2014 Number 2014 Percent
Controlled substances 1,261,000 33 1,197,000 33
Crime scene 188,000 5 170,000 5
Digital evidence 33,000 1 24,000 1
Firearms/toolmarks 132,000 3 142,000 4
Forensic biology casework 239,000 6 296,000 8
Forensic biology from samples taken from individuals convicted/arrested 1,027,000 27 904,000 25
Impressions 11,000 -- 7,000 --
Latent prints 274,000 7 301,000 8
Questioned documents 12,000 -- 9,000 --
Toxicology 606,000 16 554,000 15
Trace evidence 47,000 1 41,000 1
All requests 3,830,000 100% 3,646,000 100%

Fingerprint evidence can be instrumental when trying to place a person at a crime scene, such as from the knife used during the assault in the opening case scenario. As seen in exhibits 2 and 3, requests for fingerprint analysis have only increased. Other than DNA, fingerprints are one of the most common types of evidence that can link a person to an assault. NIJ has a rich research and development portfolio focused on new technologies related to the development of latent prints, as well as studies related to the accuracy and reliability of fingerprint examinations.[22]

See “What You Can’t See Might Solve the Case”

In certain cases of sexual assault, chemical examination of condom lubricants may also prove surprisingly valuable, as many assailants commit serial crimes and routinely use condoms to avoid leaving DNA evidence. NIJ recently funded a project at the University of Central Florida to improve the characterization and classification of condom lubricants recovered in sexual assault cases and build databases of lubricant mass spectra and infrared spectra that will be available for use in casework.[23] This type of analysis can help determine if the person used a condom and, in some cases, forensic scientists can compare the lubricant recovered from the victim with condoms seized from the suspect.

Additionally, analyzing the various materials used during an assault can provide important information; for example, physically matching a piece of tape collected from a bound victim with evidence recovered from a suspect. Recognizing that duct tape is commonly used in abductions, homicides, and the construction of explosive devices, NIJ provided funding for researchers at the University of California, Davis to perform a statistical evaluation on matching the torn and cut ends of duct tape. They examined 1,800 torn tape specimens and 400 cut tape specimens and concluded that the mean accuracy for correctly matching specimens ranged from 98.58 to 100 percent for torn tape and from 98.15 to 99.83 percent for cut tape.[24] NIJ also awarded Florida International University a grant to evaluate and validate the scientific reliability of chemical methods for profiling tapes. Using a collection set of more than 250 tapes (duct and electrical), researchers are applying rigorous analytical methods to examine the variations within and between rolls of tape to understand the relation to tape manufacturing and distribution.[25]

Finally, body fluid identification of stains (such as differentiation of semen, saliva, vaginal fluid, and menstrual blood) can be critical in identifying probative evidence and corroborating the events of an assault. Serological methods used for body fluid identification can provide more meaningful information about the nature of the crime. Even in the absence of definitive DNA results, confirmation for the presence of body fluids — semen or saliva, for example — in a victim or on a particular piece of evidence may prove significant in building a case or corroborating events. This evidence may provide a prosecutor with the source attribution information needed to connect specific forensic evidence with testimonial accounts. For example, if a victim asserts that an assailant bit her neck and ejaculated on her jeans, the body fluid identification process would allow one to assert that the fluid collected from the victim’s neck was saliva and biological material on the victim’s clothing was, in fact, semen and not from casual contact.[26] Body fluid evidence can also be significant in instances where a suspect states that the presence of blood was a result of consensual sex with a partner in menses. In these cases, identification of the stain as venous blood might refute this claim and instead support that a violent act had occurred.

As with all science, body fluid identification can be advanced. For example, the power to more accurately differentiate one body fluid from another may be increased, meaning that presumptive or preliminary identifications can be moved toward more confirmatory conclusions. Also, the time and effort required to complete testing could be decreased and sample consumption and destruction could be minimized. Currently, all commonly used testing methods involve exhausting a portion of the evidence sample to test for each body fluid presumed to be present. NIJ continues to support and fund innovative research to address the need for new tools to better identify body fluids while minimizing the consumption of evidence.

See “Body Fluid Identification.”

Empowering Criminal Justice Stakeholders and Victims Through Forensic Science

Forensic evidence as collected and analyzed is unbiased, as the science does not judge. Science is about revealing facts. For example, finding a fingerprint at the scene of a crime does not imply guilt or innocence; it is offered only to identify that a person was at a particular location or touched a particular item at some point, not whether he or she committed a crime. Any inferences drawn depend on the criminal justice practitioner using the information. Solving sexual assault crimes — bringing rapists to justice and supporting victims — is much more complicated than simply testing SAKs.

All aspects of criminal justice practice — from the initial investigation through adjudication — can benefit from DNA and non-DNA forensic evidence. However, processing and analyzing forensic evidence in a timely and efficient manner are critical. Delays in testing make the use of forensic evidence impractical during the investigative phase; therefore, many law enforcement officers do not realize the full potential of all forensic evidence as a tool for developing new leads in investigations.[27] For example, NIJ’s National Best Practices for Sexual Assault Kits: A Multidisciplinary Approach emphasizes the importance of the identification, collection, and preservation of all evidence, not just DNA, from victims and crime scenes. The forensic science community has a responsibility to provide investigators with the answers they need, when they need them. As technology and testing methods advance, new tools may become available, throughput will increase, and the benefits of forensic analysis on various types of evidence may be better realized earlier in investigations.

Within this complex process, the forensic evidence itself affects many of the decision points, including but not limited to decisions to arrest, referral for and filing of charges, plea arrangements, and court outcomes. The justice system has come to rely a great deal on various types of evidence to help link a suspect to a crime scene and to the victim, but often in sexual assault cases, forensic evidence is also used to corroborate or refute statements of fact. That is, the findings from forensic testing are also used to substantiate a victim’s or suspect’s account. In rape cases where a DNA profile from a suspect is recovered, the findings may affirm the victim’s testimony and even empower the victim, because their story is now corroborated.[28]

It is critical to provide increased training for law enforcement, prosecutors, and judges on the application and scientific validity of forensic evidence. In addition, increasing the overall capacity of our nation’s laboratories to process all forensic evidence and advancing policies and best practices for the collection and processing of evidence are crucial to the timely and accurate testing and delivery of results to the law enforcement and judicial communities. Furthermore, the criminal justice system needs sound research that not only advances current forensic practice but also substantiates the existing validity of forensic evidence analyses, so all involved can be assured that the evidence being proffered against someone is sound. Evidence has a voice; we need to ensure that science continues to be just and unbiased and that those who are listening are trained and accountable.

For More Information

Read NIJ’s “National Best Practices for Sexual Assault Kits: A Multidisciplinary Approach.”

Read about the NIJ-FBI sexual assault kit partnership.

Learn more about rape and sexual assault.

Read about sexual assault on campus.

Read four brochures to help address issues with unsubmitted sexual assault kits in your jurisdiction.

About This Article

This article was published as part of NIJ Journal issue number 279, April 2018.

Date Published: November 9, 2017