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CHICAGO (AP) — Many people arrested in Chicago were within just a few degrees of separation from someone illegally possessing a firearm, according to a recent study

The study by Northwestern University researchers was published in the Journal of Urban Health earlier this month, The Chicago Tribune reported . It attempts to understand the city’s gun market through network science.

Researchers used arrest records, gang membership rosters, shooting data and gun recoveries to group together about 188,000 arrestees and pinpoint who possessed illegal weapons. People within these networks were usually two or three connections away from a weapon, the study said.

“That’s essentially a friend of a friend,” said Andrew Papachristos, the study’s co-author and a professor of sociology at Northwestern University. “The idea is to understand how guns move. We don’t have good data on guns, and (on) how people find and use guns in Chicago, beyond police reports and anecdotes.”

Gangs played a big role in facilitating gun access, the study said. Those in gangs were typically one or two connections away from illegal firearms, Papachristos said.

“The reason people join gangs is for protection or access to a gun to protect themselves and their friends,” he said.

Boosted enforcement efforts, such as outreach and intervention, could lead gang leaders to be more restrictive with their supply of guns, the study said.

While many efforts have focused on crime-fighting and criminal justice, studying social networks may provide better targeting for public health intervention, researchers said.