The above map and timeline was created using data from the Stanford Mass Shootings in America dataset.
It contains a comprehensive information on mass shooting from 8/2/1966 to 11/6/2016.
MSA defines mass shootings as
"3 or more shooting victims (not necessarily fatalities), not including the shooter; the shooting must not be identifiably gang, drug, or organized crime related."
Unfortunately, due to the signficant resource requirement for maintaining this dataset, the Stanford Mass Shootings in America database has been permanently suspended.
To further explore the data and methodology, click
here.