I happened to read this article I thought it was interesting. Not political either which is rare when discussing mass shootings.
https://www.latimes.com/opinion/story/2019-08-04/el-paso-dayton-gilroy-mass-shooters-data
4 commonalities were found:
1. Childhood trauma and exposure to violence.
2. They had reached an identifiable crisis in the weeks or months before, like a change in job status.
3. Most had studied other shooters.
4. All had the means to carry out their plans.
Worth a read I think.