In October 1941, unbeknownst to his superiors, Sommer helped plan an attack on seven synagogues in Paris, inspired by the 1938 pogrom, in collaboration with Eugène Deloncle. A four-week suspension of pay resulted for ""SS-unwürdigen Verhaltens" (SS-unworthy behaviour)" and he would not find promotion in the next three years. He was subsequently posted to Marseille and Nice. He is also alleged to have been a spy for Krupp.