Abstract Law enforcement’s use of chemical weapons is a threat to human and environmental health, exemplified during 2020 racial justice protests in Portland, Oregon, USA. 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 help from the community, I combined first-hand accounts, videos and photos of munitions, primary literature, and analytical chemistry to identify the weapon as gaseous ZnCl2 from Hexachloroethane (HC) “smoke” grenades. Using hierarchical Bayesian methods, I estimated that DHS deployed 26 (25 – 30; 95% CI) HC grenades. Given the toxicity of ZnCl2, that many canister could have killed hundreds of people. Although no fatalities were reported, the exposed population experienced acute, delayed, and persistent health issues. DHS’s wanton use of ZnCl2 will have lasting impacts and was identified through a community standing up for racial justice . 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.