Tawel Fan ward at Glan Clwyd Hospital, Denbighshire, was closed more than three years ago and a report found some patients were treated like animals.
It has emerged that at least seven patients' families were told treatment may have contributed to their deaths.
Betsi Cadwaladr health board said an investigation was under way.
It acknowledged the quality of care provided could have been a contributory factor to the deaths of some patients.
A review of mortality rates on the ward has never been published although it is understood it has been completed.
Relatives of one patient told BBC Wales Today they were told medical care on the ward was inadequate.
Correspondence seen by the programme included an apology from the health board to the family, who do not want to be identified.
One letter said: "Experts found that there were problems in the health care which may have contributed to the death."
It added that "the board is very much engaged in a thorough search for the truth about the Tawel Fan ward".
But the family were unconvinced lessons had been learned and said questions remained unanswered and, as far as they were aware, nobody had lost their job, let alone be prosecuted.
The scandal of Tawel Fan pushed the already troubled health board into close supervision by the Welsh Government.
It remains in special measures which costs Â£5m a year.
An initial report into what happened at Tawel Fan was published almost three years ago. Two more reports are due later this year.
One of them, being compiled by the Health and Social Care Advisory Service (Hascas), is expected to include details of a mortality review of Tawel Fan patients.
But Geoff Ryall-Harvey, who leads the patient watchdog Community Health Council in north Wales, said it should be released as soon as possible.
"It may stop this practice elsewhere," he added.
A Betsi Cadwaladr health board spokesman said: "We acknowledge that the quality of care provided could have been a contributory factor to the death of some patients.
"Whether this is the case will be established as part of the independent Hascas investigation, which is currently being carried out.
"In order to establish whether or not the quality of care contributed to any patients' death, every aspect of every patient's care has to be investigated.
"This is a complicated and time consuming process, but must be carried out in order to determine whether or not the care provided was a contributory factor to any patients' death.
"Every family involved in the investigation will receive an individual report detailing the care provided to their relative. These reports will also help inform the findings of the Tawel Fan investigation."
The quality of care on a scandal-hit ward for dementia patients may have contributed to at least seven deaths, BBC Wales can reveal.