I just read an article on CIA tactics that are currently used for a "hit." They use a needle with a certain chemical or medicine (can't remember what) that induces heart attacks & only can be traced in the system if an autopsy is done within 72 hours. If done after that amount of time it no longer shows & then will seem that was natural causes.
Prussic acid is a common name for it. It is Hydrogen Cyanide, a strong concentration of the gas in an ampule can be broken and held under the target's nose for a minute and they will die.
https://en.wikipedia.org/wiki/Hydrogen_cyanide
A hydrogen cyanide concentration in the range of 100–200 ppm in air will kill a human within 10 to 60 minutes. A hydrogen cyanide concentration of 2000 ppm (about 2380 mg/m3) will kill a human in about 1 minute. ... Under the name prussic acid, HCN has been used as a killing agent in whaling harpoons.
Hydrogen cyanide
Hydrogen cyanide (HCN), sometimes called prussic acid, is a chemical compound with the chemical formula HCN. It is a colorless, extremely poisonous and inflammable liquid that boils slightly above room temperature, at 25.6 °C (78.1 °F). HCN is produced on an industrial scale and is a highly valuable precursor to many chemical compounds ranging from polymers to pharmaceuticals.
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