Last August, Federal Prosecutors released a disturbing report that detailed the serious abuses happening at the housing unit for adolescent inmates at Rikers Island, the city's largest jail system.

In it were descriptions of guards routinely beating up 16- and 17-year-olds, often hitting them in the face and head, and inmates attacking each other. The report also said guards were poorly trained and the city rarely investigated or disciplined them for using excessive force. More than 70 fixes were recommended.  

U.S. Attorney Preet Bharara said at the time that his office was negotiating reforms with the city, but would sue if necessary. On Thursday, Bharara said, "That time has now arrived."  

Bharara noted the city had already implemented several reforms, including stopping solitary confinement within the adolescent unit, increasing staff and offering more programs for teenagers.  But he said that was not enough.

"Given the long-standing, sad state of affairs at Rikers Island, our impatience is more than understandable," he said. Suing, he said, would ensure important reforms were "enduring, enforceable and verifiable."

Instead of filing a separate lawsuit, federal prosecutors would seek permission to join an already pending lawsuit filed by Legal Aid in 2012 on behalf of both adolescent and adult inmates at Rikers Island, Bharara said. 

Legal Aid Attorney Mary Lynne Werlwas said the U.S. Attorney's investigation confirms what Legal Aid has documented and reported for years. She welcomed the involvement of federal prosecutors.

"This is a historic moment. This is the first time the federal government has announced an intention to sue New York City over the mistreatment of prisoners, " she said.

City Hall spokeswoman Marti Adams said the de Blasio administration has been implementing many of the reforms that the U.S. Attorney and Legal Aid are both seeking. "We are beginning to unwind the decades of neglect that have led to unacceptable levels of violence on Rikers Island," she said.