Secondary DNA Transfer

Secondary DNA Transfer

Estimated reading time: 18 minutes

In 1930, the French scientist Edmond Locard published a journal article in which he laid the groundwork for what would become the field of forensic science. Locard said a criminal actor will always leave traces of his or her presence at a crime scene and will always leave traces of the scene on his or her person. Locard’s Exchange Principle revolutionized police investigative techniques.

The discovery of DNA further transformed the field of forensics. DNA exists in every person at the molecular level and is highly particularized to an individual. As such, DNA evidence has become the gold standard for forensic identification in criminal cases. When a suspect’s DNA is found at a crime scene, investigators have strong, virtually unassailable proof that the suspect was there.

Or do they?

Recent developments in the field of DNA analysis are allowing investigators to identify individuals using smaller and smaller samples of material. No longer are forensic scientists limited to large drops of blood or semen stains found at crime scenes. Modern DNA testing can yield an individualized profile using as few as three or four cells.

These advances have undoubtedly allowed investigators to solve crimes that they otherwise would not have. But improvements in DNA analysis have also led to a significant problem that could signal the investigative technique has reached beyond the point of diminishing returns: secondary DNA transfer.

Have DNA, Will Travel

Law enforcement officers have long relied on Locard’s Exchange Principle when investigating crimes. Early forensic techniques were as simple as locating physical objects left or taken from the crime scene and tying them to the victim or suspected perpetrator. As scientists began to better understand human biology, forensic investigators learned that the body tends to leave traces of itself wherever it has been, including when it has been involved in a crime. Unique characteristics of these bodily remnants, such as fingerprints and semen, allowed law enforcement investigators to eliminate or pinpoint suspects with strong physical evidence.

The discovery of the human genome was a watershed moment in the development of modern forensic investigation techniques. With the advent of DNA testing, investigators have become proficient at narrowing down the sources of biological material to a statistical near-certainty. When blood, hair, or semen is found at the scene of a crime, forensic scientists can now establish a genetic profile that will exactly match only one person out of about a quadrillion.

Such evidence is highly probative of an individual’s identity and is thus valued by law enforcement investigators and prosecutors. Moreover, jurors are heavily influenced by DNA evidence. A 2008 series of studies by researchers at multiple universities found that jurors rated DNA evidence as accurate 95 percent of the time and persuasive of a defendant’s guilt 94 percent of the time.

Advances in DNA collection and testing techniques now allow forensic scientists to obtain a genetic profile from microscopic amounts of biological material. Modern DNA testing and amplification kits will generate results using as few as three human cells. Attorney Erin E. Murphy, author of Inside the Cell: The Dark Side of Forensic DNA, said that “[w]hen you consider that over 10,000 cells can fit on the head of a pin, it becomes clear that the days of testing only large, visible stains are long past.”

But while forensic scientists have become highly adept at confirming an individual’s identity through the analysis of microscopically small samples of biological material, little attention has been paid to a question that is equally important to the origin of DNA found at a crime scene: How did it get there? Prior to the late 1990s, the universally accepted answer was “the originator of the DNA left it at the scene.” That’s what has made DNA evidence the magic bullet that all forensic investigators hope to find.

However, a potentially explosive problem was discovered in 1997 when Australian forensic scientist Roland van Oorschot published a short paper titled “DNA Fingerprints from Fingerprints.” The paper detailed van Oorschot’s finding that DNA can not only be transferred from Person A’s hand to an object (referred to as primary or direct transfer) but Person A’s DNA can also be transferred from the touched object to Person B’s hand. This phenomenon, known as secondary DNA transfer, introduced a significant wrinkle into the world of forensic DNA analysis.

Murphy referred to primary transfer as “the engine that turns the wheels of justice” and secondary transfer as “the cog that causes that wheel to grind to a halt.” When we acknowledge that a person’s DNA can be found on an object that he or she never touched, we come face to face with the new reality of DNA analysis: DNA may be incontrovertible evidence of identity, but DNA does not, by itself, explain how it got to where it was found.

There has been an unconscionable lack of interest among forensic scientists to study secondary transfer. But there is little doubt that what van Oorschot discovered is accurate. Cynthia Cale and other scientists in the Human Biology Program at the University of Indiana published a study in the January 2016 edition of the Journal of Forensic Science in which they confirmed the secondary DNA transfer phenomenon.

Cale conducted an experiment in which people exchanged two-minute-long handshakes immediately prior to handling individual knives. When each knife was tested, the DNA of the person who handled it was (unsurprisingly) found in almost every case. However, the tests also found the DNA profile of the person who never touched the knife 85 percent of the time. Perhaps even more shocking was that 20 percent of the time, the non-touching person came back as the primary, and in some cases the only, contributor of DNA.

“It’s scary,” said Cale, the lead forensic DNA analysis at Strand Diagnostics. “Analysts need to be aware that this can happen, and they need to be able to go into court and effectively present this evidence. They need to school the jury and the judge that there are other explanations for this DNA to be there.”

In reality, the prosecution and its DNA expert witness will almost never proactively explain the possibility of secondary DNA transfer and that it could potentially explain the presence of incriminating DNA evidence. Accordingly, it is critical for all criminal defense attorneys to have a firm understanding of secondary DNA transfer and be capable of challenging the prosecution’s expert witness on the DNA evidence.

Associate professor Krista Latham, an internationally known forensic anthropologist who oversaw Cale’s experiment, underscored the need for more research into secondary DNA transfer.

“I think this issue has been swept under the rug,” said Latham. “There have been some holes in this kind of research, and I think that allowed people to disregard it, but this is a very well-designed project. It’s going to change the way the medicolegal system looks at the evidence.”

Other researchers have confirmed secondary DNA transfer, and study results highlight the need for a greater understanding of this insidious phenomenon. Consider the following:

  • In another van Oorschot experiment, 50 percent of a group who shared a jug of juice — but did not touch each other — wound up with another’s DNA on their hand.
  • In a 2016 study, British forensic researcher Peter Gill found DNA on 75 percent of his lab’s crime scene tools, including cameras, measuring tapes, and gloves.
  • Researchers have found that DNA can transfer through non-intimate contact from the hands of one individual to another person’s penis during urination.
  • Researchers Graham and Rutty found that in more than 50 percent of individuals tested, someone else’s DNA was on their neck.
  • Researchers have found that 1 in 5 people have traces of someone else’s DNA under their fingernails.
  • A 2016 study out of Canada found that a father’s DNA is often found on his daughter’s clean underwear, and in some cases, a fragment of his sperm is there as well (presumably from the mixture of laundry in a washing machine).
  • A study published in the International Journal of Legal Medicine found that a person could transfer DNA from a cloth that had been rubbed across someone else’s neck onto an object that he or she never touched.

A more accurate understanding of DNA transfer is leading some forensic scientists to urge caution when using such evidence in court. Latham said DNA transfer and contamination issues need to be carefully considered by scientists and attorneys and must be properly explained to juries.

“DNA never lies, right? But sometimes the only evidence they have in a case is DNA, and this emphasizes that we need to interpret the entirety of a case,” said Latham. “Even everyday people on a jury need to understand that DNA is not this magic bullet, that it needs to be interpreted just like any piece of evidence.”

Given the value placed on biological evidence, it is crucial that investigators, prosecutors, and defense attorneys consider the whole picture when evaluating DNA. Confirmation bias is a serious problem in the criminal justice system, and the very existence of DNA evidence can steer an investigation in the wrong direction. Murphy, the author of Inside the Cell, said investigators and prosecutors must resist the temptation to declare DNA a smoking gun.

“We’re desperately hoping that DNA will come in to save the day, but it’s still fitting into a flawed system,” said Murphy. “If you don’t bring in the appropriate amount of skepticism and restraint in using the method, there are going to be miscarriages of justice.”

The Curious Case of the Man Who Wasn’t There

In late November 2012, a group of men broke into the home of Silicon Valley multimillionaire Raveesh Kumra. The men tied, blindfolded, and gagged Kumra, along with his ex-wife, Harinder. The house was robbed, and the men fled the scene. They also left Kumra dead; he was suffocated by the duct tape used to gag him.

Investigators working on the case combed the scene for trace evidence. A pile of latex gloves found in the kitchen sink, still wet from a recent washing, were removed for further analysis. Kumra’s body was checked for evidence. His fingernails were clipped and scraped for testing.

Santa Clara County forensic examiner Tahnee Nelson Mehmet was able to recover enough biological material from the gloves, fingernails, and duct tape to test for DNA. She was in luck; the material produced multiple, specific genetic profiles. A check of DNA databases revealed that 22-year-old DeAngelo Austin’s DNA was on the duct tape, 21-year-old Javier Garcia’s DNA was on the gloves, and 26-year-old Lukis Anderson’s DNA was on the fingernail clippings.

Further investigation revealed that both Austin’s and Garcia’s cellphones pinged towers near the scene on the night of the murder. Investigators also learned that Austin belonged to a gang known for committing residential burglaries. Austin’s sister revealed that she had been involved in a sexual relationship with Kumra for 12 years and had recently given her brother a map of Kumra’s house. As the evidence mounted, all three men were arrested and charged with murder.

But as investigators worked to put a case together, they ran into a problem: Nothing other than DNA linked Anderson to the crime. He had one felony residential burglary on his rap sheet, but a close look at the crime revealed that it amounted to little more than Anderson breaking a window in a drunken stupor.

In fact, Anderson was an admitted drunk. His criminal history was replete with alcohol-related crimes, such as drinking in public and riding a bike while under the influence. He told his public defender, Kelley Kulick, that he didn’t do things like what he was accused of, but he was almost certainly drunk on the night of the crime.

“I drink a lot,” Anderson remembered telling Kulick. “Maybe I did do it.”

Because Anderson was charged with first-degree murder, the death penalty was a possibility. Anderson’s background, including his medical records, could be used as mitigating evidence were he to actually face the death penalty. So, Kulick had an investigator pull all of his medical records.

A cursory review of the records revealed a fact that floored Kulick: Anderson was in the Valley Medical Center on the night of the murder. He had been picked up by paramedics from the San Jose Fire Department after someone called the authorities to report a drunken man. Blood alcohol tests taken that night in the hospital indicated that he had consumed the equivalent of about 21 beers.

Kulick recalled being shocked and terrified when she confirmed that Anderson had rock-solid evidence of an alibi. “To know that you have a factually innocent client sitting in jail facing the death penalty is really scary,” she said. “You don’t want to screw up.”

Fortunately for Anderson, lead investigator Eric Lunsford was willing to listen. And when he began searching for an explanation for how Anderson could have been in two places at the same time, he was not blinded by confirmation bias. In fact, it was Lunsford who ultimately figured out what happened.

It turned out that the two paramedics who responded to the Kumra murder scene were the same ones who had transported Anderson to the hospital three hours earlier. Somehow, the paramedics brought Anderson’s DNA to the murder scene and unwittingly transferred it onto Kumra’s body. Prosecutors thought the culprit was the pulse oximeter used on both Anderson and Kumra. Kulick thought Anderson’s DNA could have been on the paramedics’ uniforms.

Anderson was released after spending almost six months in jail on charges of capital murder. Garcia and Austin were convicted of Kumra’s murder, robbery of an inhabited place, and false imprisonment.

Christopher Givens, Garcia’s attorney, wondered what would have happened had Kulick not uncovered evidence that Anderson could not have been at the scene of the crime, regardless of the presence of his DNA.

“The sad thing is, I wouldn’t be surprised if he actually pleaded to something,” said Givens. “They probably would have offered him a deal, and he would have been scared enough to take it.”

Santa Clara deputy district attorney Kevin Smith told the San Francisco Chronicle that the Anderson debacle was a one-off situation, framing secondary DNA transfer as an incidence of living in what he called “a small world.”

County district attorney Jeff Rosen echoes this sentiment and said Anderson’s case was a rarity.

“There is no piece of evidence or science which is absolutely perfect, but DNA is the closest we have,” said Rosen. “Mr. Anderson was in a very unusual situation. We haven’t come across it again.”

British researcher Peter Hill, however, sees things much differently. As a forensic scientist who works with DNA regularly, he is certain that cases like Anderson’s are more common than the Santa Clara district attorney’s office believes.

“The problem is we’re not looking for these things,” said Gill. “For every miscarriage of justice that is detected, there must be a dozen that are never discovered.”

Los Angeles-based public defender and DNA specialist Jennifer Friedman agreed that there are surely cases of secondary DNA transfer that haven’t been uncovered and highlighted the difficulties faced by defense attorneys when DNA transfer is at issue.

“Although clear cases appear to be quite uncommon, I think it’s probably more prevalent than we think,” said Friedman. “The problem is that what we don’t see frequently is the ability to definitely prove that transfer occurred.”

For his part, Detective Lunsford said his understanding of DNA was forever changed by the Anderson case. “We shook hands, and I transferred on you, you transferred on me,” said Lunsford. “It happens. It’s just biological.”

Anderson was extremely fortunate that Lunsford served as the lead investigator on his case. But undoubtedly, many detectives are not as open-minded and flexible in their thinking as Lunsford. Had one of those detectives led the investigation in Anderson’s case, it could very well have had a tragic ending. There are likely factually innocent people sitting in prison right now whose cases involved the unfortunate combination of secondary DNA transfer and investigators with tunnel vision.

Phantom Serial Killers and Zombie Murderers

In 2008, European authorities were hot on the trail of a highly prolific serial killer and burglar. The “Phantom of Heilbronn” robbed jewelers, burglarized caravans, and murdered multiple people, including a law enforcement officer.

The Phantom left forensic evidence all over the continent. His DNA was found at 40 crime scenes in Germany, France, and Austria. Police offered a large reward for his capture and spent an estimated 16,000 hours working the case. But there was one small problem with the case.

He was a she, and she wasn’t a criminal mastermind. The Phantom of Heilbronn was an elderly Polish factory worker who unwittingly contaminated the forensic swabs she manufactured. While she worked, she transferred her DNA onto the swabs. Investigators later transferred her DNA to the scene of a slew of unrelated crimes.

The Phantom of Heilbronn debacle is a prime example of the dangers of DNA contamination. Inside the Cell author Erin Murphy draws a distinction between DNA contamination and DNA transfer. Contamination, Murphy argues, can be avoided or limited by employing best practices. DNA transfer, however, is a much more insidious problem because it is utterly unavoidable. In two minutes, Murphy says, the average person sheds enough skin cells to cover a football field.

Murphy profiled a horrific murder in order to illustrate the unique problems that skin-cell DNA transfer presents to forensic investigators. In 2009, a Yale graduate student was found dead in a mechanical chase area behind a wall in a scientific laboratory. DNA obtained from the victim’s body and clothing revealed two unique genetic profiles. One person was identified as a co-worker from the lab; the other was a local convicted felon.

Unsurprisingly, investigators looked at the convicted felon first. That turned out to be a literal dead-end—the man had died two years before the murder. So, investigators wondered, how was his DNA found on the victim, including in the waistband of her underwear? Secondary DNA transfer.

The convicted felon also happened to be in construction and had worked on a job behind the lab wall years earlier. He shed skin and sweat cells back there, and because the chase was closed to traffic and adjacent to the tightly controlled laboratory environment, his cells (and DNA) were still there when the victim crashed through the small space. A DNA transfer event took place, and a man was implicated in a crime that he could only have committed as a member of the undead.

The dead man also was very likely to have been what scientists studying DNA transfer would call a “good” shedder. Some people are naturally prodigious shedders of biological material. Those with flaky, sweaty, or diseased skin are thought to be good shedders. Does this matter?

It did to David Butler, an English cab driver accused of a murder that he said he did not commit. Investigators targeted Butler when they found his DNA under the victim’s fingernails six years after she was killed. Butler denied knowing the woman and said the killer may have ridden in his cab and picked up his DNA on the day of the murder.

Butler was, in fact, a good shedder. He suffered from a severe skin condition that earned him the nickname “Flaky.” The authorities didn’t buy Butler’s explanation, despite having no other evidence tying him to the crime. The jury did, and he was acquitted of the murder after spending eight months in jail.

“DNA has become the magic bullet for the police,” Butler said after he was acquitted. “They thought it was my DNA, ergo it must be me.”

The Future of DNA Analysis: From Who to How

The ability to detect and analyze microscopic bits of biological material from DNA information has revolutionized the investigation of crimes. Locard might be proud to know that his Exchange Principle has proven to be so utterly accurate. Humans engaged in crime almost always leave traces of themselves at the crime scene.

But scientists now know that humans of the non-criminal variety also leave traces of themselves almost everywhere they’ve been and, as a function of secondary DNA transfer, even in places they have not been. As such, investigators and forensic scientists must exercise caution when arriving at conclusions based on the presence of DNA evidence at a crime scene. Perhaps Locard’s Exchange Principle should be amended: A criminal will always leave traces of his or her presence at a crime scene (and in some cases, traces of someone who was never at the scene will be there, too).

Murphy used the Yale murder case to make this point and to draw attention to the lack of scientific research into the problem of secondary DNA transfer.

“The Yale story illustrates how important it is, now that we have the capacity to link perpetrators to an offense by the presence of their DNA in criminal evidence, to understand completely how easy it is for such cells to appear in a sample with which the DNA donor has never come into contact,” Murphy wrote. “Absent a good alibi—in this case, the irrefutable proof of his prior death—the worker might have ended up implicated in the crime. His familiarity with the space, along with his prior record, might have been used against him to prove that he had special knowledge of a good place to dispose of the body.”

“Unfortunately,” Murphy continued, “we still know very little about DNA transfer, and one major entity that funds DNA studies—the National Institute of Justice—has not expressed much interest in learning more.”

There is no need to hypothesize about whether secondary DNA transfer could wrongly implicate someone in a crime, of course. Lukis Anderson’s case settled that question. What remains to be seen is whether and how investigators, prosecutors, and forensic scientists will integrate the knowledge of the secondary DNA transfer phenomenon into criminal investigations and prosecutions.

Forensic scientist Suzanna Ryan of Ryan Forensic DNA Consulting said the solution is to use DNA for nothing more than what it is—direct, physical evidence that biological material belongs to a specific person. DNA should not be used as the sole piece of evidence establishing a person’s presence at a crime scene.

“While the advancements in DNA testing are overall fantastic and fascinating, the DNA analyst, investigator, and prosecution must be aware of the limitations of the testing in terms of determining how the trace DNA arrived at a location and how long it might have been there,” said Ryan. “No longer can it be assumed that if your DNA is located at a crime scene, you must have been present, and you probably committed the crime. Instead, investigators must look at other evidence surrounding the case, both circumstantial and physical, and considerations must be given to the fact that the DNA … could have arrived on the evidence innocently or it could have been transferred via a secondary transfer event.”


Additional sources: Bolivar Paula-Andrea, Tracey, Martin, McCord, Bruce. “Assessing the Risk of Secondary Transfer Via Fingerprint Brush Contamination Using Enhanced Sensitivity DNA Analysis Methods.” Journal of Forensic Sciences. January 2016. Vol. 61, No. 1, doi: 10.1111/1556-4029.12911; Cynthia M. Cale, Madison E. Earll, Krista E. Latham, Gay L. Bush. “Could Secondary DNA Transfer Falsely Place Someone at the Scene of a Crime?” Journal of Forensic Sciences. January 2016. Vol. 61, No. 1, doi: 10.1111/1556-4029.12894.