HCN is evolved when metal cyanides such as NaCN or KCN are treated with acids. This is one method by which prisoners in US jails are executed - they are strapped into a chair in a sealed, airtight room, and concentrated acid is dropped onto a large amount of a metal cyanide salt.
KCN is "Potassium Cyanide". From the CDC FactSheet on Cyanide :
* Hydrogen cyanide, under the name Zyklon B, was used as a genocidal agent by the Germans in World War II.
* Reports have indicated that during the Iran-Iraq War in the 1980s, hydrogen cyanide gas may have been used along with other chemical agents against the inhabitants of the Kurdish city of Halabja in northern Iraq.
Do you have a link where you obtained this information? This is interesting that Iraq has a history of using hydrogen cyanide specifically.