“A large container of a white powdery substance”
Ruled out the coca and poppy products?
Cops have field test kits for those. So if they don't yet know what is for certain, and suspect cyanide, it's very likely not those. Or crystal meth either.
Cyanide is said to have a bitter almond taste and odor, although some people can't smell or taste it, depending on their genetics. Surely someone smelled that smell which is why they "suspect" cyanide.