Unexplained bruising on a newborn baby was not properly investigated by medical staff in the North, a review said.
It took three visits to hospital emergency departments to discover fractured ribs and bleeding to the brain of baby Francis, the Safeguarding Board for Northern Ireland said.
Bruising had been noticed even before the child was discharged from hospital after he was born prematurely and treated in intensive care.
The infant had been in residential care with his mother, whose family was known to social services, and her partner.
The report said: "It is of concern that different hospital-based medical and nursing staff noticed bruising to Francis, but did not explore whether these might have been caused through non-accidental injury rather than having a medical cause.
"Insufficient use was made of child protection specialists in both medicine and nursing, who could have helped staff to see the bruises to Francis within the wider context of the child's social circumstances.
"There must be acceptance among disciplines that robust challenge, as the occasion demands it, is a healthy state.
"When physical trauma to Francis was still not considered a potential explanation for the child's bruises by hospital medical staff, other disciplines were not prepared to sufficiently challenge this view."
The answer to who or what caused the multiple rib fractures and bleeding to Francis' brain, and precisely how the injuries occurred, remains the subject of speculation, the report said.
The baby suffered the injuries through physical trauma, which were eventually diagnosed following scans after a number of days and repeated admissions to emergency departments.
Francis' mother and her partner were the main carers when the injuries were incurred.
The mother's family had been known to social services and other community professionals during her own childhood in relation to allegations of physical abuse, neglect and emotional abuse when support services had been provided on an intensive basis.
Francis' mother and her siblings were eventually placed in care.
The report said in a non-mobile infant the cause of Francis' injuries had to be either an accidental or non-accidental injury.
"Inquiries did not produce a plausible accidental cause.
"Francis' mother and her partner have continued to deny that they either individually, or, as a couple, ever physically abused Francis.
"In the absence of any explanation that Francis suffered an accidental injury, it therefore remains probable that the injuries were caused by a non-accidental physical trauma."
Francis suffers from a range of long term physical and cognitive disabilities, and is currently in state care.
The report said: "It remains a grey area as to exactly what extent Francis' various disabilities were caused by injury as a result of major trauma, an infection during pregnancy or a possible genetic syndrome."
A spokeswoman from NSPCC Northern Ireland said it was absolutely imperative that professionals ask hard questions if there are any doubts about a child's injuries.
"This is especially important in cases where the victim is too young to speak out and their only hope lies with those in positions of authority.
"Child abuse is completely unacceptable and we would urge neighbours, relatives and friends to be the eyes and ears which protect a child.
"Anyone who has concerns should contact the NSPCC helpline on 0808 800 5000 where our trained, experienced counsellors will be able to help."