An inquiry into the care of dementia patients on a troubled mental health ward in north Wales has been described as a “whitewash” after it said there was no institutional abuse.

The findings of an in-depth investigation into the treatment of patients on Tawel Fan ward at Glan Clwyd hospital in Denbighshire, contradicted those of an earlier inquiry.

The ward was closed in December 2013 and two years later an inquiry uncovered “institutional abuse”, which may have violated the human rights of patients.

A new investigation by the independent Health and Social Care Advisory Service did not substantiate the previous claims and said there was a “good overall general standard” of care on the ward.

It carried out more than 100 interviews with families and staff and examined over half a million pages of information including police transcripts, medical records, staff records and corporate records.

Darren Millar, the Conservative assembly member for Clwyd West, said: “Many will describe today’s report into Tawel Fan as a whitewash and will question the independence of the process which has led to its publication.

“Any organisation with links to the Welsh government or the Labour party involved in sensitive work of this kind should have a duty to disclose that to stakeholders upfront.

“The failure to do so in this case has given rise to a serious breach of trust and undermined the confidence of the Tawel Fan families that this was a genuine quest for truth.

“We are now almost four-and-a-half years on from the closure of the ward and yet, instead of answers, the Tawel Fan families have even more questions about the care of their loved ones.”

He added that the “Tawel Fan families and the people of north Wales deserve better” and called for “a cross-party assembly inquiry to ensure that we get to the bottom of what happened once and for all.”

Angela Burns, the Welsh Conservative party’s health spokeswoman, said: “When compared with the initial work undertaken by Donna Ockenden, who exposed ‘institutional abuse’ on the ward, the findings in today’s report cause concern of a different nature.

“Transparency and honesty are vital in all areas of public service delivery – particularly when scandals hit our most treasured public service.”

The Welsh health secretary, Vaughan Gething, demanded that the health board makes further improvements to services but urged caution over jumping to conclusions about the findings.

“The investigation had a much wider remit and, unlike the previous report, was able to access a comprehensive set of documentation, including clinical records, and draw in specific mental health expertise,” he said.

“This is a very substantial report that warrants further careful reading and consideration.

“Whilst this will be very difficult day for both families and staff who were involved or affected by the investigation, I would hope that these findings can act as a catalyst to the lifting of a dark shadow that has extended over mental health services in north Wales for a number of years.”

The health board said that, although the inquiry had revealed “systemic organisational weaknesses”, there had since been “substantial improvements” to services.

In a joint statement, the board’s chairman Dr Peter Higson and chief executive Gary Doherty said: “The investigation found the overall standard of care on the ward to be generally good and found no evidence to support the view that patients suffered from deliberate abuse or wilful neglect.

“However, it found that some patients did not receive the standard of care that we would expect across our services.”

They added: “ We are clear that we have much more to do to make improvements across all of our adult services – not just mental health services.”