The Surprisingly Imperfect Science of DNA Testing

Years later, none of it — not 84-year-old Eleonora Knoernschild’s bloody body on the shag carpet, not the torn bedspread twisted around her neck, not the junk heaped on her corpse so abundantly that only her left foot poked out, not three decades of detective work — none of it would matter as much as the cheese wrapper.

The day after Knoernschild was killed on Nov. 4, 1984, the local newspapers didn’t mention the cheese wrapper at all, nor the knee-high stocking that would also be of great consequence in the trials that would take place 30 years later.

Instead, the newspapers reported how Knoernschild’s premature death had been discovered: Her daughter, 59-year-old Doris Wines, had been walking to the neighborhood donut shop that Sunday morning. Wines lived just a few doors down from her mother in St. Charles, Mo., an affluent St. Louis suburb on the western bank of the Missouri River. She stooped to pick up the newspaper on her mother’s lawn. As she went to drop it off, she saw that the window in the front door had been broken. She sprinted home and called 911.

That afternoon, police told reporters that Knoernschild had been murdered and her home ransacked. The motive was likely burglary, they said, but so far there were no witnesses or suspects.

Detective Mike Harvey of the St. Charles Police Department was assigned to the case. Harvey was a Vietnam vet with a flair for convincing criminals to rat each other out. He would devote considerable reflection over the next 30 years to the death of Eleonora Knoernschild. Long after it waned in departmental memory, he was still following leads. Even after he retired in 2009, Harvey thought about her death. So when an old friend in the county prosecutor’s office called and asked if he’d like to come out of retirement to work on cold cases for them, he did not hesitate. The Knoernschild files were the first he opened.

That was in April 2010, when the cheese wrapper had not yet gained its prominence in the case, although it had been in the custody of law enforcement for many years. Detectives found the rectangle of translucent plastic on Knoernschild’s linoleum floor next to a frozen Yankee pot roast. Over the next 27 years, the cheese wrapper sat inside a manila envelope in the St. Charles Police Department’s property room.

In November 2010, Harvey asked the county forensics lab to test it and 14 other items from the crime scene for DNA. One day the following summer, DNA technician Dan Fahnestock slid the cheese wrapper from its envelope. Fahnestock, half of the county’s two-person biological forensics lab, examined the wrapper. It was covered in silver dust, a relic of fingerprint testing it had been submitted to years before.

He wiped the wrapper with six cotton swabs and placed them each in solution to dissolve any human cells they had collected. He ran the solution through a series of machines tasked with detecting, isolating, and amplifying DNA. For each of the samples, the robots produced an electropherogram, a chart with a series of flat, squiggly lines punctuated by spikes, like the EKG of a failing heart or a seismograph recording tiny earthquakes. Several of the swabs returned nothing useful, but one produced a chart with a handful of peaks, each representing a genetic marker.

Fahnestock examined the chart. It wasn’t ideal. The amount of genetic material wasn’t exactly abundant, and the DNA had decayed considerably since 1984. Some markers couldn’t be seen at all.

Conservative technicians might have stopped there, deciding the sample was too compromised to analyze. But Fahnestock continued, piecing together a profile. That profile pointed to someone Harvey had suspected for years — St. Charles native son, Brian McBenge, an ex-boyfriend of Knoernschild’s granddaughter. Six weeks later, a DNA test of a knee-high nylon stocking found behind Knoernschild’s house implicated his younger brother, Cecil McBenge.

The DNA evidence would serve as the basis for prosecuting the brothers for first-degree murder. It was a crime Harvey believed they had gotten away with for decades. But the cheese wrapper may be evidence of something else entirely: the ambiguity of a science most people never doubt.

The man Harvey says killed Eleonora Knoernschild strode to the sturdy plastic table in the visiting room where I was seated. His goatee was sandy grey, same as his hair.

He placed a stack of papers between us: The 296-page deposition of Dan Fanhestock. In preparation for our meeting, Brian had reread the deposition, and on three sheets of yellow legal paper, in tidy print, listed dozens of page numbers where he spotted holes in Fahnestock’s DNA analyses of the cheese wrapper and stocking. He started to hand me the list, but a guard stopped him. Prisoners are forbidden from passing papers to visitors.

Brian sighed and forged ahead, flipping through the pages. He pointed to a question his attorney had asked Fahnestock: Whether Fahnestock could say with certainty that Cecil or Brian were the individuals who left the DNA on the cheese wrapper or the knee-high stocking.

Brian looked at me. “And he says no.”

Fahnestock’s answer was a simple acknowledgement that even at its best, DNA science is not absolute. To Brian, it was proof that DNA cannot be trusted.

In the three decades since DNA emerged as a forensic tool, courts have rarely been skeptical about its power. When the technique of identifying people by their genes was invented, it seemed like just the thing the justice system had always been waiting for: Bare, scientific fact that could circumvent the problems of human perception, motivation, and bias.

Other forensic sciences had taken a stab at this task. Lie detector tests, ballistics, fingerprinting, arson analysis, hair examinations — all aim to provide evidence independent of the flawed humans wrapped up in an investigation. But those methods were invented by law-enforcement agencies eager for clues; it is now well established that their results are not always sound. With few alternatives, police and courts spent most of the 20th century hammering away at justice with the rubber tools of traditional forensics.

DNA was different. It came up through science, which began, in the 1950s, to unravel the ways the double helix drafts our existence. When DNA profiling led to its first conviction in a U.S. courtroom in 1987, DNA had already vaulted through the validating hoops of the scientific method. Soon it was accompanied by odds with enough zeros in front of the decimal to eliminate reasonable doubt.

“In the three decades since DNA emerged as a forensic tool, courts have rarely been skeptical about its power. When the technique of identifying people by their genes was invented, it seemed like just the thing the justice system had always been waiting for: Bare, scientific fact that could circumvent the problems of human perception, motivation, and bias.”

Today, most of us see DNA evidence as terrifically persuasive: A 2005 Gallup poll found that 85 percent of Americans considered DNA to be either very or completely reliable. Studies by researchers at the University of Nevada, Yale, and Claremont McKenna College found that  jurors rated DNA evidence 95 percent accurate and between 90 and 94 percent persuasive, depending on where the DNA was found. That faith could be shaken, but only when lawyers made a convincing case that a lab had a history of errors.

Otherwise, the mere introduction of DNA in a courtroom seemed to stymie any defense.

“A mystical aura of definitiveness often surrounds the value of DNA evidence,” the studies’ authors wrote.

In many cases, this aura is deserved. The method is unequivocal when it tests a large quantity of one person’s well-preserved genes, when it’s clear how that evidence arrived at a crime scene, and when the lab makes no errors in its work.

But those are not circumstances enjoyed by every criminal investigation. Take the case of Kerry Robinson of Georgia. Robinson was implicated, in part, when two analysts concluded his genes may be present on the victim’s vaginal swabs. The jury convicted, and Robinson received a 20-year sentence.

Greg Hampikian, a biology and criminal justice professor at Boise State University and director of the Idaho Innocence Project, was a defense expert in the trial and felt sure the analysts had reached their conclusion because of unconscious bias: They knew a great deal about the case, including that the detectives believed Robinson was guilty. To test his suspicions, Hampikian and cognitive neuroscientist Itiel Dror of University College London sent the DNA data to 17 other analysts and asked them to interpret it without any information about the case. Only one agreed with the original analysts.

Despite these results, the Georgia appeals court declined to overturn the conviction, stating that “as long as there is some competent evidence, even though contradicted … we must uphold the jury’s verdict.”

Because DNA is more reliable than other forensics, scientists have shrugged off suggestions that it could fall victim to the vagaries of bias. But Dror noted that much DNA analysis involves interpretation. With interpretation comes subjectivity, and with subjectivity can come error.

“DNA results can be in the eye of the beholder,” Dror said.


Police officer John Young was the first to arrive at Knoernschild’s house. His partner conducted a perimeter check while Young stepped over the splintered glass on the porch. The door was locked, so Young reached through its broken window and opened it. He stepped inside and found himself in the dining room.

Two cabinets and a desk had been emptied; papers and pill bottles littered the floor. Kitchen cupboards were open and food cast around. Young approached a bedroom door, where a sign warned against smoking because an oxygen tank was in use. To the right, a bare mattress. To the left, a dresser emptied of its contents. On the floor, a heap of clothes and paper. He stared at it. Knoersnchild’s foot was sticking out.

He dug through the debris. Under a blue gown, he found her face. Blood ran out of her mouth and down her right cheek, forming a pool. A tube tethered her face to a nearby oxygen tank. The fringe of her bedspread encircled her neck. The coroner would conclude that blunt force trauma had killed her.