The de Blasio administration is trying to get rid of solitary confinement for young adults who are imprisoned on Rikers Island, but fulfilling that promise has run into some unexpected problems, raising some concerns at one oversight agency. NY1's Courtney Gross filed the following report.

The oversight board for the city's jails is putting the de Blasio administration on notice.

In a letter issued Thursday, the Board of Correction said the city is violating its standards after Rikers Island officials failed to eliminate solitary confinement for young adults by a June 30 deadline. The delay, the letter went on, caused "serious concerns."

It's an unusual rebuke from an under-the-radar oversight agency. It came a week after de Blasio officials quietly asked for a six-month extension to overhaul how it was housing and handling the 18- to 21-year-old jail population.

In its own letter, the city's correction commissioner argued his agency has had problems consolidating all these young inmates into one facility on Rikers Island.

Within the last month, the number of alarms in this one jail has nearly doubled. On one day, there were 26, or more than one per hour. Serious and violent incidents, like slashings, have spiked. Young adults are jamming items into cell doors and breaking them.

In a nutshell, it argued, the department could not put all of these young adults together under one roof and eliminate solitary confinement safely at this time. It has already ended it for 16-, 17- and 18-year-olds.

"We've been moving aggressively to end it for the next group, the 19- to 21-year-olds," said Mayor Bill de Blasio. "We have to put additional steps in place to make that work as well as we want to."

Now, the federal monitor overseeing reforms on Rikers Island has endorsed this delay. He has called it sound correctional practice.

But that did not dissuade advocates, who, like the Board of Correction, are raising serious concerns.

"I don't know if it's becoming more violent or not, and I don't know who is responsible for that. I don't think the solution is locking people people up for 23 or 24 hours a day," said Jennifer Parish of the Urban Justice Center.

"Every day that someone spends, especially a young person, in isolation, that harm is going to carry with them for the rest of their lives," said Riley Doyle Evans of Brooklyn Defender Services.

The Board of Correction will officially vote on the delay next week.