The city's juvenile jails are extremely violent and unsafe, the result of a deeply ingrained culture of violence in which guards routinely violate constitutional rights of teenage inmates and subject them to "rampant use of unnecessary and excessive force," the federal government said in a scathing report released Monday.

The report, the result of a nearly three-year Justice Department investigation into violence at three Rikers Island juvenile jail facilities, recommended significant reforms to almost every aspect of how young offenders are treated.

It identified problems that occurred between 2011 and 2013 that also likely hold true for adult inmates, including poor staff training, inadequate investigations, an ineffective management structure and the overuse of solitary confinement, particularly for mentally ill inmates.

"Indeed, while we did not specifically investigate the use of force against the adult inmate population, our investigation suggests that the systemic deficiencies identified in this report may exist in equal measure at the other jails on Rikers," the government attorneys from Manhattan wrote.

In past cases investigated by the Justice Department's civil rights division, federal authorities work with local officials to reform the jails and reserve the right to sue if they feel reforms are not being done.

Mayor de Blasio has promised to reform the nation's second-largest jail system, with an average of 11,500 inmates held at any time. In March, he appointed Joseph Ponte, a longtime corrections official from Maine with a reputation as a reformer.

Prosecutors acknowledged that de Blasio and Ponte inherited many of the problems, which have been pervasive for years despite some small reform-minded undertakings during the final years of former Mayor Bloomberg's tenure.

But the theme repeated throughout their 80-page report was that violence permeates the three Rikers facilities for 16- to 18-year-olds.

The investigators found particular problems in the main jail that houses young inmates, the Robert N. Davoren Complex, where new correction officers are assigned to "one of the most combustible environments at Rikers, ill-equipped to cope with adolescents who are often belligerent and suffer from a wide range of mental illnesses and behavioral disorders," assistant U.S. attorneys Jeffrey Powell and Emily Daughtry wrote.

They found that jail guards too often struck inmates in the face and head with closed fists while staff and potential witnesses are instructed not to report uses of force.

The consultant used by the federal prosecutors said that in reviewing hundreds of correctional systems, he had never seen such a high rate of head shots, use-of-force rate and such pervasive inmate-on-inmate violence.

In one August 2013 case detailed by the investigators, four unidentified inmates suffered broken noses, perforated eardrums, head trauma and facial injuries during a "brutal use of force" involving multiple guards in a trailer where school classes are held — and again later when they were handcuffed in a clinic holding area.

In that case, the inmates told the federal consultant they were assaulted after getting into an argument with a jail guard. Correction officers disputed that account, writing in reports to their bosses that the inmates jumped an officer and confronted others who came to the guard's aid.

There is no video of the encounter and, nearly a year after it occurred, corrections investigative reports were not completed, the federal government lawyers noted. What's more, in the initial use-of-force reports, the officers used similar wording of how the fight went down, "suggesting that the officers may have colluded with each other to ensure their reports were consistent."

That case is representative of others, the federal attorneys found, noting that when uses-of-force occur, their reporting is widely inconsistent and subsequent investigations are regularly incomplete, untimely and often inconsequential.

Four officers were involved in 50 to 76 use-of-force incidents at Davoren complex between 2007 and 2012.

"The officer with the highest number of uses of force during the six-year period (76) was disciplined only once during this time," they wrote. "Most of the others were disciplined once or twice, and some never."

Last fiscal year, there were 489 adolescent inmates, down from 682 in fiscal year 2013, when the average jail stint for 16- to 18-year-olds was 76 days. More than half had a mental health diagnosis. When they break jailhouse rules, such as refusing to obey orders or being verbally abrasive, they are sent to punitive segregation, a 23-hour lock-in essentially the same as solitary confinement.

In one 21-month period, an average of 150 inmates received such punishment each month, resulting in a total of 143,823 days in solitary.

At Rikers, the federal attorneys detailed dozens of changes that should be made, including moving all the youth inmates to a facility off Rikers supervised by experienced, better-trained guards; installing more video cameras; instituting a zero-tolerance policy for not reporting uses of force; and developing top-down training for staff.