The Gun Violence Archive is an online archive of gun violence incidents collected from over 7,500 law enforcement, media, government and commercial sources daily in an effort to provide near-real time data about the results of gun violence. GVA is an independent data collection and research group with no affiliation with any advocacy organization.
https://www.gunviolencearchive.org/reports/mass-shooting is a good page to start with. there are also 0pages which show “Last 72 hours.” So information on GVA is rather moving and informational.