Among more than a dozen specific cases of brutality detailed in the report was one in which correction officers assaulted four inmates for several minutes, beating them with radios, batons and broomsticks, and slamming their heads against walls. Another inmate sustained a skull fracture and was left with the imprint of a boot on his back from an assault involving multiple officers. In another case, a young man was taken from a classroom after falling asleep during a lecture and was beaten severely. Teachers heard him screaming and crying for his mother.

“For adolescent inmates, Rikers Island is broken,” Mr. Bharara said at a news conference announcing the findings. “It is a place where brute force is the first impulse rather than the last resort, a place where verbal insults are repaid with physical injuries, where beatings are routine while accountability is rare.”

Image Rikers Island emergency services entering the jail's juvenile detention facility. Credit... Julie Jacobson/Associated Press

The federal investigation was conducted by the civil division of the United States attorney’s office. Officers involved in specific incidents were not identified by name. But the report listed more than 10 pages of remedial measures, and it warned that if the city did not work cooperatively to develop new policies and procedures, the Justice Department could bring a federal lawsuit asking a judge to order the imposition of remedies. Mr. Bharara said the city had 49 days to respond to the findings.

Joseph Ponte, the city’s new correction commissioner, said in a statement that his agency had “cooperated fully” with the Justice Department, and would work with it to carry out whatever changes were “appropriate and feasible.”

The report, which covers 2011 through the end of 2013, touched on many of the same issues raised in an investigation by The New York Times into violence by guards at Rikers, particularly against inmates with mental illnesses, published last month.

The Times article documented 129 cases in which inmates of all ages were seriously injured last year in altercations with correction officers, including several attacks that were also singled out in the report.