Brutal attacks by correction officers on inmates - particularly those with mental health issues - are common occurrences inside Rikers [Island], the country's second-largest jail. Reports of such abuses have seldom reached the outside world, even as alarm has grown this year over conditions at the sprawling jail complex. A dearth of whistle-blowers, coupled with the reluctance of the city's Department of Correction to acknowledge the problem and the fact that guards are rarely punished, has kept the full extent of the violence hidden from public view. But [in a four-month-long investigation, The New York Times has] uncovered details on scores of assaults through interviews with current and former inmates, correction officers and mental health clinicians at the jail, and by reviewing hundreds of pages of legal, investigative and jail records. Among the documents obtained ... was a secret internal study completed this year by the city's Department of Health and Mental Hygiene, which handles medical care at Rikers, on violence by officers. The report helps lay bare the culture of brutality on the island and makes clear that it is inmates with mental illnesses who absorb the overwhelming brunt of the violence. The study ... found that over an 11-month period last year, 129 inmates suffered serious injuries" - ones beyond the capacity of doctors at the jail's clinics to treat - in altercations with correction department staff members. The report cataloged in exacting detail the severity of injuries suffered by inmates: fractures, wounds requiring stitches, head injuries and the like. But it also explored who the victims were. Most significantly, 77 percent of the seriously injured inmates had received a mental illness diagnosis.