Almost 250 former detention centre inmates have come forward to make allegations of historic abuse at the hands of staff.
Cleveland Police confirmed the investigation into events in the 1970s, 80s and 90s at what was Kirklevington detention centre near Yarm is continuing.
Earlier this year it said two men aged 70 and 61 had been arrested, the older man on suspicion of rape. They were released without charge.
The force said Operation Magnolia has now heard from 248 complainants.
Kirklevington housed boys aged 14 to 17 who had been sentenced to between three weeks and four months detention. These days they would be likely to receive sentences in the community.
Former inmates have told of a regime of immediate violence by staff upon arrival, cold baths and beatings.
One of them, David Ginnelly, told the BBC Inside Out programme earlier this year: "I don't doubt that what I did deserved punishment, but not brutality, not to that extent, not at 14 years old."
Solicitor David Greenwood, who represents a number of complainants, said: "The men I am helping complain of serious assaults from staff at Kirklevington.
"Most of these assaults were physical in nature but a small number were sexual assaults.
"The investigation by Cleveland Police is welcomed and will hopefully shed light on the brutal treatment meted out to young offenders."
Durham Police are carrying out the country's biggest investigation into historic sexual abuse, at the now-closed Medomsley Detention Centre, near Consett.
So far 1,254 people have told detectives they suffered physical or sexual abuse there.
Kirklevington is now a prison for adults.