Nineteen more graves have been found at a Florida reform school where one former inmate said \"the devil was hiding behind every tree,\" scientists said Monday. A team from the University of South Florida in Tampa has been investigating the graveyard at the Arthur G. Dozier School for Boys in Marianna, NBC News reported. In an interim report, they said they have found at least 50 grave shafts. The school closed in 2011, after investigators said boys had been abused and even killed at the school, which opened in 1900. Previously, officials had said 31 boys were buried on the school grounds, most of them killed in a fire and in an epidemic, both of them early in the school\'s history. The researchers said some of the grave shafts may hold the remains of more than one child. Some of the graves were in the woods, outside the cemetery\'s formal boundaries. Roger Dean Kiser, who was an inmate at the school in the 1950s, wrote \"The White House Boys -- An American Tragedy,\" a title that came from a building on the grounds where some of the worst abuse was inflicted. He described the school as a \"concentration camp for little boys\" and said \"a devil was hiding behind every tree, every building and even behind every blade of manicured grass.\" Erin Kimmerle, an assistant professor of anthropology, said there may be other undiscovered burial grounds at the school. She said the one the team has been exploring appears to hold the remains of black students and, in the early 1900s, there would probably have been at least one more for white students.