They found that in approximately 8 out of 10 cases, the perpetrator was not a lawful gun owner but rather in illegal possession of a weapon that belonged to someone else. The researchers were primarily interested in how these guns made their way from a legal purchase — at a firearm dealer or via a private sale — to the scene of the crime.

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"All guns start out as legal guns," Fabio said in an interview. But a "huge number of them" move into illegal hands. "As a public-health person, I'd like to be able to figure out that path," he added.

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More than 30 percent of the guns that ended up at crime scenes had been stolen, according to Fabio's research. But more than 40 percent of those stolen guns weren't reported by the owners as stolen until after police contacted them when the gun was used in a crime.

One of the more concerning findings in the study was that for the majority of guns recovered (62 percent), "the place where the owner lost possession of the firearm was unknown."

"We have a lot of people with a lot of guns," Fabio said, referencing statistics on the large number of guns in circulation. "And some of them aren't keeping track of them for different reasons — maybe because they have a lot of them and they don't use them that often."

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A number of factors could lead to legal firearms entering the black market. Owners could misplace them, or they could be stolen — either through carelessness on the owner's part (leaving a gun in an unlocked car, for instance) or determination on the part of thieves.

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It's also likely that many guns on the black market got there via straw purchases — where a person purchases a gun from a dealer without disclosing that they're buying it for someone else. This is illegal under federal law. One potential sign that straw purchasing is a factor in the Pittsburgh data: Forty-four percent of the gun owners who were identified in 2008 did not respond to police attempts to contact them.

The top-line finding of the study — that the overwhelming majority of gun crimes aren't committed by lawful gun owners — reinforces a common refrain among gun rights advocacy groups. They argue that since criminals don't follow laws, new regulations on gun ownership would only serve to burden lawful owners while doing little to combat crime.

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But Fabio's research suggests that this strict dichotomy between "good guys" and "bad guys" isn't necessarily helpful for figuring out how to keep "good" guns — those purchased legally — from getting into "bad" hands. And there may be modest, non-burdensome ways to help keep guns in the hands of the good guys.

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For instance, 10 states plus the District of Columbia have laws in place requiring gun owners to report the theft or loss of firearms to law enforcement, according to the Law Center to Prevent Gun Violence, a group that advocates for stronger firearm regulations. But in the majority of states, no such law is in place.

Additionally, past research has demonstrated that a small fraction of gun dealers are responsible for the majority of guns used in crimes in the United States. A 2000 report from the Bureau of Alcohol, Tobacco and Firearms found that in 1998, more than 85 percent of gun dealers had no guns used in crimes trace back to them. By contrast, 1 percent of dealers accounted for nearly 6 in 10 crime gun traces that year.

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The firearms bureau knows exactly who these gun dealers are — but they're not allowed to share that information with policymakers or researchers due to a law passed by Congress in 2003. As a result, solutions for stanching the flow of guns from these dealers to crime scenes remain frustratingly out of reach for public-health researchers.

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"There's not much federal funding out there to do research on firearm and firearm safety," Fabio said. As a result, "there's not a lot of good research out there. The process of getting it done has been hindered by a lot of limits on academics and how they can do firearms research."