This study uses open source information to examine school shootings in the United States for the 1990-2016 period. We innovatively created a national-level database to address the gaps in existing research and identified 652 school shootings. These shootings included 473 intentional shootings (encompassing 354 with known offenders and 119 with unknown perpetrators), 102 suicide only shootings, 73 accidental discharges and 4 legally justified shootings. Most school shootings were committed outside of the school building (e.g., school yard), by non-students, during non-school hours and were sometimes motivated by non-school issues such as gang disputes. Almost 56% of the intentional shootings resulted in no deaths and mass homicide shootings were outliers. No clear time trend emerged. Importantly, proportionally more of the non-juvenile offenders committed fatal shootings. While the vast majority of attacks targeted high schools, those against elementary schools were more deadly. We outline how these findings could aid policymakers, and highlight issues that future research could address with these data. We also describe our open source collection procedures, the amount and type of information uncovered, and how we assessed their quality. We aim to set a standard for more transparent open source data collection processes and enhance the data's rigor to provide important context to the larger policy discussions about school shootings.