It was in his capacity as an SS company commander that he gained notoriety, being responsible for the 27 May 1940 massacre of British prisoners-of-war at Le Paradis in the Pas-de-Calais.

Ninety-nine members of the 2nd Battalion of the Royal Norfolk Regiment, who had surrendered to his unit in a cattle shed, were stood in front of the barn wall, and Knöchlein ordered two machine-guns turned on them, followed by bayoneting and shooting any apparent survivors.

Two of the prisoners, privates Albert Pooley and William O'Callaghan, managed to escape the massacre, but the remaining 97 were hastily buried along the barn wall.

According to the historians Williamson Murray and Allan Millet: "The company commander, Hauptsturmführer (Captain) Fritz Knochlein, lined the prisoners up against a barn wall and machinegunned the lot.

[3] Albert Pooley, who was subsequently taken prisoner along with O'Callaghan, made it a personal mission to hunt down Knöchlein and have him charged for war crimes.