That year, he began working at an institute led by Virgil Madgearu, where he remained until 1947. During the rest of the decade, Zilber, characterized as "cultured, voluble and ubiquitous", and known in the cultural circles of Bucharest, led an underground life as a member of the banned Romanian Communist Party working to destroy "bourgeois democracy", an informer and a spy, but also lived the seemingly normal life of a bourgeois desirous of prosperity. Following the Coup of August 1944, when the Communist Party was made legal, he became the closest collaborator of his friend Lucrețiu Pătrășcanu, who became Justice Minister. Zilber was arrested in 1948, together with Pătrășcanu, and imprisoned following a show trial. He was released in 1964. He died in 1978. Following incineration at the Cenușa crematorium, his ashes were taken to Cișmigiu Gardens, where they were scattered under a bush near the bust of George Panu, among the magnolias he often admired. His memoirs appeared posthumously in 1997, some years after the 1989 fall of the regime.