Marcus Klingberg, the Israeli biologist who served 15 years in prison after he was convicted of spying for the Soviet Union, died in Paris early Monday. He was 97 years old.Klingberg’s health had been poor in the latter stages of his life. He is considered the most important spy that the Soviets employed in Israel.A biology professor who reached the rank of lieutenant colonel in the IDF’s Medical Corps, Klingberg went on to work for the Israel Institute for Biological Research in Ness Ziona. The institute is considered one of the most top secret intelligence facilities in the country.According to foreign media reports, the research center is where Israel manufactured biological weapons and poisons used by the Mossad in its assassination missions.Klingberg succeeded in climbing the ranks of the institute, reaching the position of deputy manager. By dint of his job title, he had access to all of the site’s most sensitive documents and information.He was arrested in 1983 by authorities after his cover was blown by a double agent who unbeknownst to him was also working for Israel. A military tribunal found Klingberg guilty of espionage and sentenced him to 20 years in prison.After serving 15 years, he was released on parole. One character witness who testified on Klingberg’s behalf was the former head of the Israel Security Agency (Shin Bet) Yaakov Perry, who is currently a member of Knesset with Yair Lapid’s Yesh Atid party. In 2003, Klingberg left Israel and relocated to Paris, the home of his daughter, Silvia, and his grandson.Klingberg was born in Poland in 1918 to a religious family. As a youth, he turned secular and elected to study medicine at the University of Warsaw.Following the outbreak of World War II, he left his parents, who were eventually murdered in the Holocaust along with his entire family, and moved to the Soviet Union to continue his studies.After the Nazi invasion of the Soviet Union, Klingberg enlisted in the Red Army, where he rose to the rank of major in its medical corps. His area of expertise in the military was epidemiology. During his service, he suffered wounds to his leg. After the war, he returned to Poland.In 1949, he and his wife, Wanda, a microbiologist, immigrated to Israel.Klingberg joined the IDF and later took a position with the Institute for Biological Research.During the course of his military service and his tenure in the institute, he took part in some of the most confidential projects. As a member of the ruling Mapai Party, he enjoyed a close association with the country’s leaders, including then-prime minister Levi Eshkol.While at the institute, Klingberg was twice summoned by the authorities after the Mossad and Shin Bet received information indicating that he was working as an agent for either the Soviet Union or one of the Soviet bloc countries. Klingberg refuted the claims, and he even passed a polygraph.It was only after Klingberg made contact with a Soviet agent secretly working for the Israelis that the authorities were able to pin him on espionage charges. Even though the government did not have sufficient evidence to bring charges against Klingberg in court, he eventually confessed to passing information to the GRU, Moscow’s foreign military intelligence directorate.In retrospect, Israeli authorities learned that Klingberg began working for the Russians before immigrating to Israel. Still, there are Shin Bet officials who claim that he was extorted by his Soviet handlers who reminded Klingberg that he never completed his studies in medicine due to the outbreak of World War II.Klingberg long maintained that he did in fact finish his studies and that he would often meet his handlers in the Russian Orthodox Church in south Tel Aviv and during trips to Vienna and Geneva.After his release from prison, he co-wrote a book with his attorney, Michael Sfard, in which he revealed that his wife was also a Soviet agent. As an employee of the institute, she helped her husband sneak confidential information and lab tests out of the country.