Urgent, individualized interventions to reduce firearm access, such as extreme risk protection orders (ERPOs, colloquially known as "red flag" orders), provide a rapid, focused response when risk for imminent firearm violence is high. Studies to date suggest such interventions are most commonly used to prevent suicide and are effective. Authorizing legislation has often been enacted after public mass shootings but, to our knowledge, there have been only 2 reported cases of ERPO use in efforts to prevent mass shootings. California enacted the nation's first ERPO statute, which took effect in January 2016. The authors are evaluating that statute's implementation and effectiveness and are seeking to obtain court records for all 414 cases occurring in 2016 to 2018. Based on 159 records received thus far, this article presents an aggregate summary and individual histories for a preliminary series of 21 cases in which ERPOs were used in efforts to prevent mass shootings. Most subjects were male and non-Hispanic white; the mean age was 35 years. Most subjects made explicit threats and owned firearms. Four cases arose primarily in relation to medical or mental health conditions, and such conditions were noted in 4 others. Fifty-two firearms were recovered. As of early August 2019, none of the threatened shootings had occurred, and no other homicides or suicides by persons subject to the orders were identified. It is impossible to know whether violence would have occurred had ERPOs not been issued, and the authors make no claim of a causal relationship. Nonetheless, the cases suggest that this urgent, individualized intervention can play a role in efforts to prevent mass shootings, in health care settings and elsewhere. Further evaluation would be helpful.