It's not a circular argument. It's not assuming that all people with specific characteristics will commit mass murder in a specific location without examining whether all people with the same characteristic also commit mass murder in various locations. That would be circular.
It is examining the traits of mass shooters who commit these acts in a specific location to determine what traits they may have in common.
How can include both the chosen location and the means, both of which are examined by the study.
"The Violence Project also identified five profile categories of shooters. This includes K-12 shooters, who are usually white males who plan their actions and steal guns from their family, compared with college shooters, who are more likely to be suicidal, non-white, and to write a manifesto.
Other profiled groups include shooters at places of worship, commercial locations, and workplaces." https://www.mic.com/p/new-study-on-m...ators-19366339
"This is also the first database to look closely at how many shooters obtained their guns. The majority of mass shooters use handguns (seventy-seven percent) and twenty-five percent used assault rifles. Of the known data, seventy-seven percent of shooters purchased at least some of their guns legally, thirteen percent made illegal purchases, and nineteen percent stole guns."https://www.hamline.edu/news/MassShooterDatabase/
"Peterson’s team looked closely at the mental health of the shooters and found that about 80% of them showed signs of a crisis leading up to the event.She believes that’s where a red flag law, which gives police the authority to temporarily remove weapons, could be beneficial."https://kstp.com/news/local-research...2019-/5557861/
Again, confine your comments to the topic.