Ron Suskind describes an alleged plot by al-Qaeda to release hydrogen cyanide in the NYC subway system:
In the world of terrorist weaponry, this was the equivalent of splitting the atom. Obtain a few widely available chemicals, and you could construct it with a trip to Home Depot and then kill everyone in the store.Actually, the terrorist equivalent of splitting the atom would be splitting the atom. The device Suskind describes would not and probably could not kill everyone in a Home Depot store, not least because those stores have high ceilings and are well ventilated. Suskind uses the Nazi gas chambers to demonstrate the horrors of HCN poisoning, without giving sufficient attention to the rather stark architectural differences between a gas chamber, a Home Depot store, and a subway station.
This is no way to proceed, unless your goal is to frighten people unnecessarily (while distracting them from more likely and more devastating possibilities). To put things in perspective, the sarin attack in Tokyo's subway killed 13 people; unless I'm mistaken, sarin is roughly 100 times more powerful than HCN.
Defense Tech has more.