Scientists have found 19 previously unknown grave shafts on the grounds of a notorious Florida reform school, suggesting that many more boys died there amid brutal conditions than had previously been known, the researchers said Monday.

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The Arthur G. Dozier School for Boys in Marianna, which was also known as the Florida State Reform School, closed in June 2011 after state investigators and the U.S. Justice Department's Civil Rights Division confirmed widespread abuse over many decades.