How do you find the evidence of a drug-fueled crime inside a body—years after the person has already died?

That’s the question forensic scientist Brian Andresen faced when, in 1999, the Glendale Police Department tasked him with helping them investigate the suspicious deaths of patients at Glendale Adventist Medical Center hospital. According to investigators, a notorious serial killer may have been on the loose in the hospital, a so-called “Angel of Death” injecting patients with lethal doses of the paralyzing drug Pavulon.

One year earlier, a respiratory therapist, Efren Saldivar, had confessed to the killings, which police suspected to be a number well upward of 100. But he later walked back his admittance. That didn’t stop police from going to Andresen, one of the top forensic experts around, hoping he could exhume the thought-to-be victims and test for traces of Pavulon in their bodies—and prove Saldivar’s guilt.

Now, Andresen was no slouch when it came to solving crimes. The lab he worked at, the Forensic Science Center at Lawrence Livermore National Laboratory, was called the “Lab of Last Resort” for a reason. But even Andresen wasn’t sure if he’d be able to pull usable samples from corpses that had been decomposing in cemeteries for years, let alone find the deadly drug in them.

It wasn’t until Andresen tried out a peculiar lab technique—used to test chemical weapons, of all things—that he finally found a solution to the dilemma and helped investigators bring the “Angel of Death” to justice. How Andresen utilized a unique mass spectrometry approach to catch a killer sits at the heart of this Popular Mechanics feature, and gives a behind-the-scenes look at how new forensics techniques helped solve the Saldivar case.

Before Andresen even exhumed the remains of the patients, he wanted to test various methods for extracting Pavulon. He began by taking a piece of beef liver from a nearby Safeway grocery store, mixing the tissue with the drug and then attempting to extract it. It didn’t work. Andresen then switched from testing beef to pig meat—no luck with that, either. His experiments went on for months, and with each passing test, Andresen didn’t seem to get any closer to a workable technique for extracting Pavulon.

But then Andresen came across a fascinating solution in an unexpected place: within the Forensic Science Center unit studying uses of chemical weapons for the Organisation for the Prohibition of Chemical Weapons. Using single-phase cartridges—syringe-like objects with filters inside—researchers could separate the lethal elements from other substances in chemical weapons. Maybe, Andresen theorized, that method could work in his own case.

Andresen quickly went to work, using single-phase cartridges to isolate Pavulon in decomposed test tissues, and then placed them in a mass spectrometer to confirm the presence of the drug. The method proved to be a winner. Andresen just needed to put it to use on human corpses, and hope that the technique logged similar results.

From the body of each possible victim that came into the lab, Andresen and his team took 12 tissue samples from all different parts, including the heart, lungs, kidney, stomach, and intestines. They then cut 10 grams from each sample, added a salt solution to it, and combined it in a blender before pouring the thick tissue solution into a single-phase cartridge and throwing it into the mass spectrometer.

It took months before Andresen had any luck finding Pavulon in the bodies of the deceased patients. But then came the first hit—proving Andresen’s method worked. “We were like, jumping around, going, ‘Yes, we got it!’” recounts Andresen’s lab partner, Armando Alcaraz, in the full story. Andresen and his team members would go on to find six positive cases of Pavulon, each in the remains of people believed to be Saldivar victims.

Andresen’s findings led to an indictment of Saldivar on six counts of murder and one count of attempted murder from a Los Angeles County grand jury in October 2001. In March 2002, Saldivar pleaded guilty to the seven charges, and is currently in prison serving a life sentence without parole.

Would Salvidar have ever been caught without the groundbreaking work from Andresen? According to investigators, not likely. As retired Los Angeles County District Attorney Steve Cooley says in the Pop Mech feature, the work from Livermore Lab was “the key to establishing the corpus delicti,” or the evidence necessary for police to prove a person committed a crime. By finding proof from the dead, Andresen brought the “Angel of Death” case back to life.

Dive deeper into the mind-boggling forensics innovations and the experts behind the chilling investigation into the California serial killer by reading the full story now.