The IRA murder of a schoolboy is the "most horrific" case of child abuse being considered by the Historical Institutional Abuse Inquiry (HIA) , it has been revealed.
Bernard Teggart, 15, was abducted from St Patrick's Training School in west Belfast in 1973.
He was shot in the head some hours afterwards and died in hospital.
Inquiry counsel said the killing is "the most horrific incident of child abuse to come before the HIA Inquiry".
It was revealed that the school authorities did not report the abduction to the police.
But the inquiry counsel added: "Identifying potential systems failures by those who regulated or ran a training school does not take from the fact that it was the IRA who murdered Bernard Teggart, a 15-year-old boy said to have a mental age of an eight or nine-year-old."
The teenager's father Danny had been shot dead by the Parachute Regiment in Ballymurphy in 1971, along with 10 others.
Bernard's brother John Teggart said he wants his brother's murder fully investigated.
"What we need now, we need a thorough investigation, just like an investigation of what happened to my father in August 1971," he said.
"Let's start with an investigation into the evidence that they have there. You have the RUC were dealing with it at the time - let's bring that forward.
"Whether it's independent or whether it's any other, let's bring it forward and let's be open and honest."
The inquiry was told that Bernard was abducted along with his twin brother, Gerry, who had also been taken from the school the week before.
The two brothers were taken to a number of houses and interrogated. Eight hours later Barney was found lying fatally wounded beside a roadside in north Belfast. He died some time later in hospital.
Gerry Teggart was released by the killers.
One possible reason for the murder of the teenager was that he had witnessed an IRA robbery at a beer company and told the robbers who were threatening a beer lorry driver to leave him alone.
The HIA inquiry was set up in 2013 to investigate child abuse in residential institutions in Northern Ireland over a 73-year period, up to 1995.
In total, the inquiry is expected to hear from more than 300 witnesses during the course of the public evidence sessions.
It is required to complete its hearings and all investigative work by mid-summer 2016, and has to submit its report to the Northern Ireland executive by 17 January 2017.