A threatening letter making references to poisoning, violence and repeated abuse, sent to the family of a man in disability care in South Australia, was not taken seriously enough by authorities the disability royal commission has found.
The commission has also ruled subsequent investigations into the letter by the SA Department of Human Services were deficient, unjustifiably delayed and unsatisfactory.
In a report handed down on Wednesday, it said the overall response reflected a failure to adopt a person-centred approach focusing on the man's safety and wellbeing.
"DHS failed to discharge its responsibility to take appropriate measures to find out the source of an apparently serious threat to the safety and wellbeing of a person with a disability in its care," the commission said.
"The failure to instigate a formal investigation in a timely fashion had significant consequences for the manner in which DHS handled the matter afterwards."
The findings followed evidence presented to the inquiry last year relating to a man identified as Mitchell who has autism spectrum disorder and lives with an intellectual disability.
In that evidence, the commission was told the letter had been received by Mitchell's family in March 2018 and referred to him as "the piglet".
It made references to him potentially being poisoned, having acid put in his shampoo bottle, falling down stairs, being locked up, being denied food or "going through the windscreen, seatbelt unclipped".
"This little piglet is going to be abused with cruelty, violence ... regularly and repeatedly," the letter said.
The royal commission said the seven-week delay by the DHS in conducting the first investigation meant the chances of identifying the author of the letter were less likely.
It described the inquiries as superficial and said the investigation did not systematically review or check rosters, incident report registers or other available electronic records to determine who might have been involved, despite the department considering it likely a DHS employee was the author.
The commission was also critical of the second investigation, describing it as deficient in both its methodology and scope.
The failures caused Mitchell's parents significant distress and led to them to lose trust and confidence in the DHS, it found.
In its previous submissions, the SA government conceded the initial investigation into the letter was insufficient and became "distracted" from the aim of finding the person responsible.
However, it said the department had taken the letter seriously and had acted to conduct more checks on Mitchell's accommodation complex, increase staffing levels and appoint a new supervisor.
While in response to earlier recommendations, the department confirmed an independent investigator had been appointed to conduct a further inquiry into the letter.
In other findings handed down on Wednesday, the commission said autistic man Daniel Rogers was subject to neglect while in residential care, suffering from poor personal hygiene and grooming and being forced to live in an unsatisfactory state of cleanliness.
It said an investigation into significant bruising Mr Rogers suffered was "at best cursory" and did not constitute a serious attempt to determine the cause of the injuries or the person or persons responsible.
In reference to the cases of both Mitchell and Mr Rogers, the royal commission said the DHS failed to create an environment where they were kept as safe and well-supported as they should have been.
"There has been inadequate institutional accountability on the part of DHS for the failures experienced by Mitchell, Daniel Rogers and their families," the commission said.
"This has prevented them from feeling that their concerns have been properly addressed and that DHS has taken appropriate measures to acknowledge the impact on them of DHS's failures."
