In this Podcast Extra, NPR correspondent Joe Shapiro recalls the life and legacy of Martin Sostre, someone he first reported on as a student in the 1970s. Sostre died a free man in 2015. But he spent at least nine years of his life in solitary confinement, including in the notorious Attica prison. Today, Sostre's life and pioneering prisoners' rights work is largely hidden from the public.

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