I'm confident it's white men with a history of misogyny and access to guns. The data supports that too. We love screaming about the one offs but that's smoke and mirrors.
[0] https://ucr.fbi.gov/crime-in-the-u.s/2019/crime-in-the-u.s.-...
[1] Due to how the FBI reports Hispanic ethnicity separately from race, it is difficult to calculate homicide rates for whites and Hispanics separately, so for this comparison the white+Hispanic rate will have to suffice.
That said, mass shootings are a small part of all homicide, so they give the wrong impression of overall violent crime and homicide, and I see no justification for ignoring non-mass homicide. But the media love to focus on them to the exclusion of all else.