During the First World War the Germans didn't gas the prisoners in their camps. Instead they used the prisoners' letters to their wives back home in France as a means to gas them!
"... it is said that recently the Huns resorted to the expedient of poisoning wells with cyanide of potassium.
German aviators, flying over French and Belgian cities also are reported to have dropped packages of poisoned candy in the hope of killing a few more innocent children. Also they are credited in reports received in Paris, with injecting asphyxiating gas into letters sent home by prisoners in in their camps. A woman living in the French village of La Siscotiere [sic] recently was ill for several days, after opening a letter from a French prisoner in Germany.
On the same day, it said, another woman received a letter also containing poison gas from her husband, a captive in Bavaria. Other poisoning cases of a similar nature also have been reported."