What does it take to convict a serial killer? Evidence that leads to the perpetrator might include clues from bodies, eyewitness accounts, or fingerprints on weapons. Yet the complexity of these cases often requires investigators to look deeper—to genetic material naked to the human eye.
DNA evidence can bring resolution to cold cases. Until mid-July of this year, there were no active leads for the Long Island Serial Killer, thought to be responsible for the deaths of victims found at Gilgo Beach, New York, between 1996 and 2011. But new DNA evidence gave police the proof they needed to arrest New York architect Rex Heuermann for the murder of three women whose corpses were found on the beach a decade ago and a possible lead on a fourth body. Heuermann, who through a lawyer pleaded not guilty in July, appeared in court yesterday.
While each criminal case is different, when it comes to catching a murderer, DNA is the gold standard (but not infallible). For Heuermann, DNA from a discarded pizza crust and strands of his wife’s hair linked him to the crimes. “In the case of Rex Heuermann, there is already more than enough evidence,” says Carole Lieberman, a forensic psychiatrist and expert witness. “There doesn’t have to be an actual witness to his crimes.”
As DNA technology advances, police can catch killers more quickly—and, possibly, find answers to the nearly 15,000 cold cases in the US.
Suspects from a sliver of DNA
You share about 99.9 percent of your DNA with everyone else in the world. To get around that problem, forensic experts have to focus on an extremely tiny portion of non-identical DNA, called short tandem repeats (STR). These are two to five base pairs of repeated multiple types in a single chromosome, explains Lawrence Kobilinsky, a professor emeritus of forensic science at John Jay College of Criminal Justice. STRs make up 3 percent of the entire human genome.
STRs have a high mutation rate, meaning this area varies widely from person to person. If two samples have the same pattern of base pairs, it’s a strong indicator that the DNA belongs to the one individual. “As long as you have the quantity and quality of evidence, those nuclear DNA tests come up with tremendous information for human identification,” Koblinsky says.
In Heuermann’s case, police found traces of hair on three of the victims. Some STR analyses found the hairs belonged to a female while another hair strand found on the tape used to tie up one victim, Megan Waterman, came from a male, but investigators could not gather any more information. Most of it was practically unusable. This isn’t surprising; after 13 years of exposure to the elements, environmental factors such as humidity, temperature, and contamination from bacteria or fungi likely decomposed the DNA.
Just a bit of intact genetic material, though, goes a long way. If you have about 100 picograms of DNA—roughly 50 to 100 cells—Koblinsky says that it is enough to get an STR profile. “Less than that, you’re only going to be able to do mitochondrial DNA [testing].”
Mitochondrial DNA makes up a small fraction of the total amount of DNA in a cell—16,500 base pairs out of the 3 billion in DNA. But it is more robust than other types of cellular DNA, allowing it to withstand environmental conditions that would normally contaminate other genetic material. A centimeter of hair contains enough to generate a mitochondrial DNA profile. This allowed New York City police to obtain DNA from the leftover pizza that Heuermann threw out. Police took the trash back to the lab, where it was swabbed for mitochondrial and autosomal DNA, genetic material inherited by both parents.
Mitochondrial DNA’s shortcomings
Mitochondrial DNA is less reliable than the STRs, though, because people inherit their mitochondria from their mothers. “A woman and her mother, grandmother, aunt, niece, or granddaughter will all have the same mitochondrial DNA profile,” says Koblinsky. “Even a person’s brother or sister will have the same mitochondrial profile.”
Since mitochondrial DNA cannot provide a 100 percent match to the perpetrator, prosecutors need to present some type of statistic on the likelihood the DNA from a crime scene is the same as the defendant. They might explain the chances of being wrong as one in a quadrillion or one in a thousand, for example.
[Related: DNA evidence is not foolproof]
The more mitochondrial DNA from different sources, the better. Police took evidence from each crime scene and the pizza crust, and they also sampled 11 bottles in a garbage bag Heuermann left outside his home. When swabbed and tested for mitochondrial DNA, the results resembled the DNA profile of his wife and were similar to the female hairs found in burlap sacks used to wrap the bodies. This is not a big surprise, says Koblinsky, because people tend to shed between 50 to 100 hairs a day. “It’s not unusual for hair to stick to your clothing. Possibly his wife’s hair got transferred to him and then those burlap bags where he bound the victims.”
Better sensitivity and accuracy
Advances in DNA technology are making it easier to identify and convict murderers who could strike again. Mitochondrial tests have higher sensitivity and specificity than 10 years ago, Koblinsky explains. Next-generation sequencing (NGS)—a tool for detecting the order of DNA base pairs—has improved, creating quicker and more accurate DNA profiles from genetic evidence collected from crime scenes. A new NGS method using CRISPR-CAS9, for example, was able to identify 0.1 percent of a genome sample in a DNA mixture containing other unrelated material. According to the National Institute of Justice, this technique “outperformed other NGS methods” and was able to identify 2,500 variants with an 83 percent accuracy.
AI technology is also making it easier to examine low-quality DNA samples, sift through DNA sequences on felon databases, and check for crimes committed in other states. As police dig more into Heuermann’s past, the public will learn whether the alleged Long Island Serial Killer took a decade-long hiatus, or if there are more bodies waiting to be uncovered.
Multiple lines of evidence are needed to build a case against a serial killer, as Lieberman points out. Having witnesses and establishing patterns of behavior can narrow the search for a murderer, but DNA could be the turning point between circumstantial evidence and an open-and-shut case.