The U.S. Department of Justice has criticized the New York City Department of Corrections for using excessive force and solitary confinement to punish adolescent inmates on Rikers Island.

In a 79-page report sent to Mayor Bill de Blasio, investigators detail a “deep-seated culture of violence” and call for some 70 reforms.

It was the product of a two-year investigation into conditions at three juvenile facilities on Rikers Island that found, among other things, that  “adolescent inmates are not adequately protected from physical harm due to the rampant use of unnecessary and excessive force” — particularly in areas lacking surveillance. It also said they’re not protected from inmate-on-inmate violence and are locked in punitive segregation “at an alarming rate.” Up to 25 percent were in isolation during any given day last year.

Manhattan U.S. Attorney Preet Bharara said at a news conference on Monday that “for adolescents, Rikers Island is a broken institution.”

“It is a place where brute force is the first impulse rather than the last resort; where verbal insults are repaid with physical injuries; where beatings are routine while accountability is rare; and where a culture of violence endures even while a code of silence prevails,” he said. “The adolescents in Rikers are walled off from the public, but they are not walled off from the Constitution.”

From 2011 to 2013, adolescent males between 16 and 18 reported thousands of injuries at Rikers that allegedly resulted from “staff use of force,” particularly in the Robert N. Davoren Center, the main juvenile facility where the Department of Corrections’ “green officers” are assigned.

(Under New York law, juveniles 16 and older can be charged as adults.)

In one instance last year, four teenage inmates claim officers brutally beat them, throwing punches and kicks and hitting them with radios, batons and broomsticks. Officers claim it was self-defense, arguing that an inmate pulled an officer off of a ladder and other inmates jumped in, assaulting the officer with broken mop sticks and a metal rod.

The inmates suffered serious injuries — a broken nose, a perforated eardrum, head trauma and chest wounds. Two said they blacked out. And it took too long to get the inmates to a hospital, according to a senior official who wrote in an e-mail that “this type of delay could have proven fatal.”

There’s no video footage of the brawl because there were no security cameras in the area — a trailer that housed classrooms. According to the report, the teens claim they were then taken to holding pens, handcuffed and beaten a second time. Investigators discovered several officers used the same phrasing in their written reports of the incident that occurred in August 2013, “suggesting that the officers may have colluded with each other to ensure their reports were consistent.”

Investigators suggested reforms such as transferring underage offenders to a facility off of the island supervised by more experienced guards, implementing a zero-tolerance policy for failing to report instances of force and installing more security cameras.

The Associated Press reported four officers were involved in up to 76 use-of-force incidents at the Davoren Center between 2007 and 2012. The officer with the most incidents during a six-year stint was disciplined once.

“Going forward, we will work with the city of New York to make good on our commitment to reform practices that are unfair and unjust,” U.S. Attorney General Eric Holder said, “and to ensure that — in all circumstances, and particularly when it comes to our young people — incarceration is used to deter, punish, and ultimately rehabilitate, not merely to warehouse and forget.”