A training school did not tell the RUC about the abduction of a 15-year-old boy who was later murdered by the IRA, the Historical Institutional Abuse Inquiry heard.
Bernard Teggart, who had the mental age of an eight or nine-year-old, was abducted from St Patrick's Training School in west Belfast in 1973.
The teenager, from New Barnsley Crescent in west Belfast, was taken along with his twin brother Gerry, who had been abducted from the institution the day before.
The pair were interrogated for several hours. Gerry Teggart was released but Bernard was shot in the head. He was found seriously injured by the side of a road in north Belfast and later died in hospital.
It was revealed during yesterday's hearing that the training school's authorities did not report the abduction to the RUC.
Inquiry counsel said the killing is "the most horrific incident of child abuse to come before the HIA Inquiry".
The inquiry heard: "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."
In 2004, Bernard's sister told The Irish Times that her brother had seen the IRA hijacking of a beer lorry in west Belfast and had told one of the gunmen to leave the lorry driver alone.
Later that year, the IRA apologised to the Teggart family for the murder.
In a statement, the paramilitary group said it had investigated the killing following a request from the family.
"We offer our sincere apologies to the Teggart family for the pain and grief we have caused. They killing should not have happened," the statement read.
Bernard's death came just two years after his father Danny was shot dead by the Parachute Regiment in Ballymurphy in 1971, along with 10 other civilians.
The teenager's brother John Teggart, who campaigns on behalf of the Ballymurphy families, last night called for an investigation into Bernard's death. "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 abuse inquiry is investigating historic child abuse in residential institutions in Northern Ireland.
It is due to submit its findings to the executive by the beginning of 2017.
