Gardai are expected to question at least seven Christian Brothers, some retired, who worked in the Artane Industrial School in Dublin after complaints by former pupils about very serious sexual abuse and other physical violence.
A team of eight experienced detectives, working under a detective sergeant, has been set up to handle the Artane investigation.
It is understood that at least 20 men who were in the orphanage before it closed in 1969 have made statements to detectives.
The investigating team, based at Clontarf station, is understood to have been contacted by several other former pupils who also wish to make statements of complaint.
One of the brothers who worked at Artane was arrested and questioned by the detectives on Wednesday. He was released later, and a file is being compiled for submission to the Director of Public Prosecutions (DPP).
The school was the biggest industrial school for boys in the State, and thousands of boys were accommodated there from the turn of the century until it closed almost 20 years ago.
Sources close to the investigation say some of the statements given by former pupils have contained harrowing details of sexual abuse and violent assault.
The alleged abuses relate to a period from 1952 to 1968, and on Wednesday the Garda Press Office issued a public appeal to any former residents and staff of the school who wish to make complaints to contact the detectives at Clontarf Garda station, telephone (01) 853-1000.
The investigation began several weeks ago after gardai in the North Dublin Division received complaints from men who had stayed at the industrial school.
In recent years serious allegations have been made about Artane by former pupils. In 1995 one issued civil action against the Christian Brothers Order naming four brothers who had served in Artane and alleging that, as a child in their care, he was subjected to regular sexual abuse.
Some brothers in the industrial school had a reputation for corporal punishment of the boys, most of whom were orphans or abandoned children, in their care.
In a statement last night, the Order said: "We want to make it very clear that we are co-operating fully with the Garda investigations being carried out as a result of complaints that they have received.
"The Brothers deeply regret the hurt that they may have caused in any school or institution with which they are or have been associated.
"In March of this year the Christian Brothers issued a message and apology. This message was designed to reach out to people who had been hurt and we encouraged people to come forward.
"We welcome the fact that people have since done so. Again we deeply regret the very fact that there is a need for this and we say sorry to anyone we have wronged.
"We realise that any journey towards any form of reconciliation is going to be long and painful, but we are determined to continue on this journey."