Klingberg renewed his contacts with Soviet intelligence and began spying in the 1950s. According to his indictment, which was based on his confession, he began spying in 1957. From then to 1976, Klingberg passed information about the activities in Israel's chemical and biological fields. Israel's foreign and domestic intelligence agencies, Mossad and Shin Bet, began to suspect Klingberg of espionage in the 1960s, but brought no results. While at the Institute for Biological Research, he was twice summoned by the authorities due to suspicions of him being a foreign agent. Klingberg refuted these claims, and in 1965 he passed a polygraph test, reportedly due to the fact that his interrogators simply asked the wrong questions, as they suspected he was spying for Polish rather than Soviet intelligence.