The scientific analysis of Patrick Pursley’s gun was key evidence in the murder trial that put him behind bars. Nearly two decades later, an expert who initially examined the weapon said he was no longer certain that it was used in the killing.

Dan Gunnell, of the Illinois State Police Joliet Forensic Science Laboratory, was one of the experts who examined Pursley’s gun in 1993. Back then, Gunnell was convinced the 9 mm Taurus semi-automatic handgun fired the two bullets that killed Andrew Ascher during a robbery on April 2, 1993 in Rockford.

Despite no eyewitness testimony placing Pursley near the scene, he was sentenced to life in a maximum security prison. Pursley maintained his innocence throughout his 23 years behind bars. After numerous appeals, a judge ordered in 2011 that state police retest Pursley’s gun.

After testing the gun again, Gunnell told the court that he “would not testify the same way today.” Upon review, other state experts also called the results inconclusive.

A judge vacated Pursley’s conviction in March 2017, which means he is free on bond while a panel of judges decides if the state can try him again. On Wednesday, those judges will hear arguments about ballistics evidence, officially called firearm and toolmark examination.

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State police maintain their conclusions are accurate enough to give as testimony in court, but other experts have argued the test results can be unreliable. If the judges say prosecutors can’t try Pursley again, that could allow other lawyers to question ballistics evidence in many other cases and open the door for those convicted on similar evidence to seek a retrial.

How do ballistics tests work?

Ballistics is the study of guns and bullets. Specifically, it’s based on the premise that every gun leaves a nearly unique set of marks on bullets and casings.

If police have a gun they think was used in a crime, they can send it to a state laboratory for testing. An expert then fires that gun in the lab and compares the markings on the test rounds with the ones found at the crime scene. There is a broad range of potential identifiers.

For example, examiners look at things like firing pin impressions (seen below).

Under a high-magnification microscope, examiners also can look for markings that were etched onto a bullet as it moved through the barrel of a gun (seen below).

“If we find that mark pattern repeated on the evidence bullet … we can conclude that that bullet was fired by the same gun,” said Jim Kreiser, a retired firearms and toolmark examiner who worked with the Illinois State Police Springfield Forensic Science Laboratory from 1968 until 2002.

Kreiser, who said he performed hundreds of exams per year, described the job as a “pattern matching exercise.” However, he noted that “sometimes [examiners] can’t tell. That’s when we have to say that this bullet could not be identified nor eliminated as having been fired from this firearm.”

Controversial science

Not all experts are confident in the science. A National Academy of Sciences report concluded that firearm analysis is often used in court “before the techniques have been properly studied and their accuracy verified.”

And in 2016, a government-appointed advisory group of leading scientists and engineers released a study that cited serious concerns about the underlying science of ballistic analysis. The study said the practice relies on circular logic and its “conclusions are subjective.”

“It’s basically a guess,” said Jon Loevy, an attorney representing Pursley’s case in civil court. “I’m sure there’s a lot of ballistics examiners that swear by the science, but that doesn’t make it scientific.”

Specifically, Loevy questioned the subjective nature of the analysis.

“Why do we believe science? It’s not just because it’s a guy who says ‘I’m a scientist’ giving it — it’s because you can replicate results with studies,” Loevy said. “Well, there’s no study that corroborates that this shell casing looks like that shell casing — it’s just some guy’s opinion.”

But Kreiser, the retired state police expert, said there are measures in place to insure accuracy. He said examiners undergo aptitude tests, blind tests, and other quality assurance controls.

“If it’s true that it wasn’t a scientific process, and that we didn’t know what we were doing, how come we get all these tests right all the time?” Kreiser said.

Illinois State Police did not answer questions about their firearm examinations, quality assurance techniques, and its use in prosecutions.

‘A rollercoaster ride from hell’

Pursley could be the first person in Illinois to successfully use the post-conviction testing of ballistics evidence to get out of prison, his lawyers said. There may be “hundreds” of others convicted on similar ballistics examinations, Loevy said.

Pursley, 52, has been out on bond and living with his fiance since April 2017. Despite his successful appeal, Pursley said he is still worried the ballistics analysis could land him back behind bars.

“In my 23 years in prison, I saw many, many men get [retrials], and they wound up being found guilty again,” Pursley said. “It’s like a rollercoaster ride from hell … I’m still very much facing a new trial for a murder I didn’t commit.”

He said if he’s granted his freedom, he plans to establish an organization that focuses on using forensic science to free the wrongfully incarcerated.

“A lot of people are locked up for crimes they didn’t commit,” he said. “And science is the key to freedom and liberty.”

Max Green is a producer at WBEZ. Follow him on Twitter at @MaxRaphaelGreen.