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Also from the business insider:
"There is no broadly accepted definition of a mass shooting. The Gun Violence Archive defines a mass shooting as a single incident in which four or more people, not including the shooter, are "shot and/or killed" at "the same general time and location."
Another source suggests that death rate from "Islamic terrorism" in the US is somewhere around 1 in 3,500,000. If you presume other types of extremism are similar, there is still a significant difference from the mass shooting rate.
https://www.businessinsider.com/how-many-mass-shootings-in-a...
https://politicalscience.osu.edu/faculty/jmueller/since.html