The cyanide compound used was potassium cyanide and it is the cyanide ion, CN−, which is the toxic component. The cyanide ion is a good ligand and complexes to Fe(III) inside cells thus shutting them down.
It seems possible that the potassium cyanide used by Yussupov could have been stored in damp conditions. Atmospheric carbon dioxide reacts with moisture to form the weak acid carbonic acid:
CO2(g) + H2O(l) →H2CO3(aq)
This is a weak acid (pKa6.3) but it is strong enough to react with potassium cyanide to form hydrocyanic acid (pKa9.3) and potassium hydrogencarbonate.
H2CO3(aq) +KCN(s) →KHCO3(s) + HCN(g)
Hydrocyanic acid (hydrogen cyanide) is covalently bonded and the molecule is a gas at room temperature. The gaseous hydrocyanic acid would have escaped into the air leaving behind the harmless potassium hydrogencarbonate. This is a white powder and would be visually indistinguishable from the potassium cyanide.