Expert witnesses can no longer testify in court that a bullet involved in a crime was fired from a particular gun based on firearm identification techniques, the Supreme Court of Maryland ruled Tuesday.

Firearm identification asserts guns leave unique patterns and marks on bullets, the opinion said, and that experts can use them to link casings from a crime scene to a specific gun.

Chief Judge Matthew Fader, who wrote the ruling, noted that the majority doesn’t question that firearms identification is generally reliable. He wrote that it can be helpful to a jury in identifying whether patterns and markings on “unknown” bullets or cartridges “are consistent or inconsistent with those on bullets or cartridges known to have been fired from a particular firearm.”

Fader also noted that it’s possible that experts who are asked the right questions or have the benefit of additional studies and data may be able to offer opinions “that drill down further on the level of consistency exhibited by samples or the likelihood that two bullets or cartridges fired from different firearms might exhibit such consistency.”

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“However, based on the record here, and particularly the lack of evidence that study results are reflective of actual casework, firearms identification has not been shown to reach reliable results linking a particular unknown bullet to a particular known firearm,” Fader wrote.

Three justices dissented.

The identification method is commonly used as evidence in court and has been “the backbone of countless prosecutions,” according to Jeffrey Gilleran, chief attorney of the Forensics Division of the Maryland Office of the Public Defender. Gilleran estimates it may have been used as evidence in hundreds of cases per year in Maryland, and that it has been used for decades.

But, according to the ruling, studies have questioned the ability of ballistics experts to produce “accurate, repeatable and reproducible” results, prompting Maryland’s highest court to rule that firearm experts cannot use it to testify a specific bullet came from a specific gun.

Experts can, however, testify that patterns and markings on bullets “are consistent or inconsistent” with bullets fired from a particular gun, the opinion said.

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The ruling was made after a man appealed his conviction of first-degree murder and related handgun offenses in the killing of his roommate in Prince George’s County. The case involved testimony from an expert who testified bullets from the crime scene were fired from the defendant’s gun, which Maryland’s Supreme Court said should not have been allowed.

The court ordered the man’s convictions be vacated and that he have a new trial.

In a statement, Brian Zavin, chief of the Appellate Division at the Maryland Office of the Public Defender, called the decision “a step in the right direction.”

“Our criminal legal system has allowed unvalidated and unreliable methods to be admitted at trial for too long. Countless individuals have been convicted on the basis of quasi-scientific evidence that does not withstand scrutiny,” Zavin wrote.

Experts will examine the weight and caliber of a bullet or shell casing to narrow down the model of gun that fired it. Then, they will look for certain characteristics that are known to more specific types of guns. And finally they’ll look for individual variations in a specific gun.

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Variations can happen when a bullet goes through a barrel of a gun and picks up certain deformities that are unique to it, said Thomas Mauriello, a forensic consultant and senior lecturer at the University of Maryland’s Department of Criminology and Criminal Justice.

“Simply put, when two objects touch each other, they take on characteristics of each other,” he said.

When police recover bullets or shell casings, they can test-fire a gun and compare the recovered bullets to the test-fire bullets by studying these characteristics, according to the National Institute of Standards and Technology.

But some studies argued that results are not reliable or repeatable, which Gilleran said are “basic requirements of any scientific method.”

Gilleran pointed to a study conducted by the Federal Bureau of Investigation which found that when different experts examined the same bullets, they disagreed at relatively high rates.

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“People deserve to have confidence in the legal system, and so I do applaud the court for taking the stand for science,” he said.

Mauriello, though, said in some cases, evidence from firearm identification can be strong — such as a situation in which you have the whole bullet and the variations on its surface match up all the way around the bullet, he said.

“We have to remember that the jury always — doesn’t matter what the evidence — the jury decides for themselves whether they want to believe it or not. But the Maryland court in this case is saying the jury can’t even hear that,” he said. “Well, I don’t agree with that.”

In a statement, the Maryland Office of the Attorney General said the court “issued a narrow decision” and that examiners can still testify about the consistency of bullets found at crime scenes and a particular firearm.

“Future cases will also provide the opportunity to amplify the scientific record and evidence to demonstrate developments and analysis of the science that would support experts linking bullets definitively to a specific firearm,” the statement said.

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The Association of Firearm and Tool Mark Examiners, which created the primary methodology for firearm identification, wrote in a statement that instances in which testimony based on firearm identification is limited “are of concern to AFTE and is inconsistent with the research that has been conducted over many decades.”

The majority opinion, the statement from the organization’s board of directors said, “focused on the qualifications that they believed should accompany an opinion; however in doing so, they provided no guidance on what this qualification should be, leaving the lower courts to make these scientific decisions and usurping the role of jurors in our legal system.”

According to Gilleran, the implications of the ruling remain to be seen, because it isn’t entirely clear what it means that experts can testify that a bullet is “consistent or inconsistent” with a particular gun. The question is how trial courts will interpret that, and what guardrails will be put on expert witnesses, he said.

The Associated Press contributed to this report.