Abstract Law enforcement’s use of chemical weapons is a threat to human and environmental health, exemplified during 2020 Black Lives Matter (BLM) protests in Portland, OR, where city, county, state, and federal agencies have deployed various chemicals for over 100 days. In July, US Department of Homeland Security (DHS) agents used an exceptionally toxic and unknown weapon to quell free speech in support of Black lives and against federal presence. With significant support from the community, I combined first-hand accounts, print media, videos and photos of munitions, primary literature, and analytical chemistry to identify the weapon as gaseous Zinc Chloride (ZnCl2) from Hexachloroethane (HC) “smoke” grenades. I used hierarchical Bayesian methods to estimate that DHS deployed 26 (25 – 30; 95% CI) HC grenades in July, which, given the toxicity of ZnCl2, was enough to kill 235 (156 – 306) people. Although no fatalities have been associated with this ZnCl2 release thus far, it has led to acute, delayed, and persistent major health issues in the exposed population, including bystanders. Further, symptoms overlap with influenza/COVID-19 and Zn is not regularly assayed in patients, hampering medical diagnoses. Given alignment of novel mass symptoms with prior case histories, however, ZnCl2 is certainly the causal agent and has created an ongoing human and environmental health crisis extending well beyond the protests’ footprint in downtown Portland. DHS’s wanton use of ZnCl2 against protesters will have lasting impacts for decades and was identified through a community of civilians standing up to say Black Lives Matter. Significance Statement The US Army Chemical Warfare Service invented Hexachloroethane (HC) smoke screens after World War I, but by the mid-1950s armed services around the world were well-aware of HC’s acute and chronic toxicity. The “smoke” produced by HC devices is gaseous Zinc Chloride (ZnCl2), a known lethal compound that induces heavy metal poisoning (“metal fume fever”); chemically burns dermal, bronchial, and gastrointestinal epithelia; and bioaccumulates. Despite well-established impacts, US Department of Homeland Security (DHS) agents deployed HC in Portland Oregon to quell Black Lives Matter protests in July 2020. I use a hierarchical Bayesian model that combined multiple observation streams to compile community-collected video and photographic evidence and estimate that DHS deployed enough grenades to kill or maim hundreds of people.