The Los Angeles police officers closing in on a serial killer known as the Grim Sleeper needed just one last piece of evidence. To capture the man, who had eluded them for more than two decades while murdering more than a dozen women, a daring detective went undercover to obtain the final piece of the puzzle: a half-eaten pizza crust.
Dinner leftovers might not be the first thing that comes to mind when we think of smoking-gun murder evidence, but in the age of modern forensic science, a tiny drop of saliva on a discarded pizza slice can be just as convincing as a literal smoking gun. The crust ended up being a key to the Grim Sleeper’s 2010 arrest and prosecution.
Forensics—the application of scientific and technological advances to crime investigation—has revolutionized law enforcement in the last several decades. High-tech tools now allow detectives to detect fingerprints that would have previously gone unnoticed, to analyze and compare fibers with unprecedented precision and to use massive databases to make instant connections between seemingly disparate pieces of evidence.
The most stunning advances have come in the field of DNA, which has helped police to crack cases that seemed unsolvable and attorneys to free prisoners who were wrongfully imprisoned.
When police first started using DNA analysis in the ’70s and ’80s, they would need an amount of blood the size of a quarter to get a good sample, says Peter Massey, a former Connecticut detective who’s now head of the undergraduate program at University of New Haven’s Henry C. Lee College of Criminal Justice and Forensic Science. “Now it’s down to less than a pinhead,” he says.
With “touch DNA” police can now obtain a sample after a suspect simply has physical contact with a surface, and police have much greater ability to use “degradated samples,” DNA evidence that could be months or even years old, says Massey.
Advances in DNA investigation also mean that analysis that once took three or five days are down to as little as 90 minutes, and can be done with a portable testing unit, says Massey. Police can even zero in on a suspect without a direct DNA match, by making a connection through familial genetic resemblance. That was how police caught the so-called Grim Sleeper, as crime reporter Christine Pelisek detailed in her 2017 book of the same name.
Police had recovered DNA evidence from saliva found on the Grim Sleeper’s victims’ breasts, but it didn’t match any known samples, Pelisek writes. Then detectives ran the sample through a new familial DNA database in 2010, and got a hit: a man related to the killer. Police discovered that the man’s father happened to live smack in the middle of where all the victims had been found. Soon, a detective posing as a pizzeria busboy was able to grab the DNA sample police needed to identify the father—sanitation worker Lonnie D. Franklin Jr.—as the killer.
As DNA analysis has improved, so have forensic tools like fiber analysis, which played a role in the arrest of serial killers like Ted Bundy, Wayne Williams and Richard Evonitz. For years, scientists have been refining a process called Raman spectroscopy, which can precisely identify fibers left at a crime scene. Scientists shine a laser at a sample and examine the photons that bounce back, revealing a signature spectrum, characteristic of a very specific type of polymer. This can allow police to link fibers at a crime scene to a particular kind of shirt a suspect might have been wearing.
Bill Schade, a latent print examiner in Florida with 40 years of experience, says the biggest advances in his field have been in the sensitivity and clarity of his instruments. Classically, fingerprints were collected from a crime scene using light or dark powder to make them show up, a process that worked best only on non-porous materials, he says. But these days, investigators can use chemicals to develop prints left on a piece of paper or even a wooden two-by-four, or make trace finger oils luminesce with special light sources, he says.
These advances, combined with computer-assisted analysis and national databases, mean police can make cases from smaller and smaller fingerprint fragments. Those factors came together this spring to make an arrest in a Florida murder case from 1974.
Police charged a popular New Orleans street performer known as “Uncle Louie” with the decades-old homicide of a convenience-store clerk, thanks to the nationwide Automated Fingerprint Identification System. The same process, then brand new, helped catch the Night Stalker in 1985.
Despite improvements in other areas, DNA remains the gold standard in forensic science, and more uses are being discovered. The latest frontier is using DNA evidence to create a computer-generated image of what a suspect might look like.
Steven Armentrout, the head of Parabon NanoLabs, the company that’s developing the technology, explains that Parabon uses a computer algorithm trained to recognize certain traits—eye and hair color, skin color, face shape—in a DNA sample and creates an image, the same way a police sketch artist might use eyewitness accounts. “DNA at a crime scene can serve as a genetic witness,” says Armentrout.
Police are only just beginning to exploit this latest application of DNA evidence, but it’s already proving useful. Detectives in Louisiana using the technology made an arrest this summer in a previously unsolved murder from 2009.
Read more: When Forensic Science Gets It Wrong