The man considered the Soviet Union's most successful spy in Israel before being unmasked more than 30 years ago has died in Paris at the age of 97, his daughter said Tuesday. Marcus Klingberg was sentenced to 20 years in prison in 1983 for having passed information to Moscow on Israeli biological weapons research. "He was a Communist who acted out of conviction and gratitude to the Red Army for having allowed him to fight the Nazis who massacred his entire family in Poland," daughter Sylvia Klingberg told AFP.