They did something like this again in 2021, and wouldn't even disclose what the tracer substance was. https://www.nbcnewyork.com/news/local/homeland-security-d

They did something like this again in 2021, and wouldn't even disclose what the tracer substance was. https://www.nbcnewyork.com/news/local/homeland-security-deploying-non-toxic-gas-in-nyc-trains-next-week-for-terror-readiness-study/3316701/ https://www.nbcnewyork.com/news/local/homeland-security-depl... ("Non-Toxic Gas to Be Deployed in 100+ NYC Locations, Including Transit, In Bio-Attack Readiness Test") https://news.ycombinator.com/item?id=28889222 https://news.ycombinator.com/item?id=28889222 (5 comments)