The NHS has failed to investigate the unexpected deaths of more than 1,000 people since 2011, according to a report obtained by BBC News.
It blames a "failure of leadership" at Southern Health NHS Foundation Trust.
It says the deaths of mental health and learning-disability patients were not properly examined.
Southern Health said it "fully accepted" the quality of processes for investigating and reporting a death needed to be better, but had improved.
The trust is one of the country's largest mental health trusts, covering Hampshire, Dorset, Wiltshire, Oxfordshire and Buckinghamshire and providing services to about 45,000 people.
The investigation, commissioned by NHS England and carried out by Mazars, a large audit firm, looked at all deaths at the trust between April 2011 and March 2015.
During that period, it found 10,306 people had died.
Most were expected. However, 1,454 were not.
Of those, 272 were treated as critical incidents, of which just 195 - 13% - were treated by the trust as a serious incident requiring investigation (SIRI).
The likelihood of an unexpected death being investigated depended hugely on the type of patient.
The most likely group to see an investigation was adults with mental health problems, where 30% were investigated.
For those with learning disability the figure was 1%, and among over-65s with mental health problems it was just 0.3%.
The average age at death of those with a learning disability was 56 - over seven years younger than the national average.
Even when investigations were carried out, they were of a poor quality and often extremely late, the NHS England report says.
Repeated criticisms from coroners about the timeliness and usefulness of reports provided for inquests by Southern Health failed to improve performance, while there was often little effort to engage with the families of the deceased.
Key findings from the report
- The trust could not demonstrate a comprehensive systematic approach to learning from deaths
- Despite the trust having comprehensive data on deaths, it failed to use it effectively
- Too few deaths among those with learning disability and over-65s with mental health problems were investigated, and some cases should have been investigated further
- In nearly two-thirds of investigations, there was no family involvement
The reasons for the failures, says the report, lie squarely with senior executives and the trust board.
There was no "effective" management of deaths or investigations or "effective focus or leadership from the board", it says.
Even when the board did ask relevant questions, the report says, they were constantly reassured by executives that processes were robust and investigations thorough.
But the Mazars investigators said: "This is contrary to our findings."
The culture of Southern Health, which has been led by Katrina Percy since it was created in 2011, "results in lost learning, a lack of transparency when care problems occur, as well as lack of assurance to families that a death was not avoidable and has been properly investigated," the report says.
The report was ordered in 2013, after Connor Sparrowhawk, 18, drowned in a bath following an epileptic seizure while a patient in a Southern Health hospital in Oxford.
An independent investigation said his death had been preventable, and an inquest jury found neglect by the trust had contributed to his death.
Responding to the report's findings, Connor's mother, Sara Ryan, said the entire leadership of Southern Health had to go.
She said: "There is no reason why in 2015 a report like this should come out. It's a total scandal. It just sickens me."
'Considerable measures' taken
The report's authors, who spent months looking into how Southern Health failed to investigate so many deaths, seem to concur.
They say: "We have little confidence that the trust has fully recognised the need for it to improve its reporting and investigation of deaths."
In response, Southern Health NHS Foundation Trust said it accepted its processes "had not always been up to the high standards our patients, their families and carers deserve.
"However, we have already made substantial improvements in this area over a sustained period of time."
It added: "These issues are not unique to the trust and we welcome the opportunity to shine a spotlight on this important area.
"Though the trust continues to challenge the draft report's interpretation of the evidence, our focus and priority is on continuing to improve the services we provide for our patients."
It said that when the final report was published by NHS England, it would review the recommendations and make any further changes necessary.
An NHS England spokesman said: "We commissioned an independent report because it was clear that there are significant concerns.
"We are determined that, for the sake of past, present and future patients and their families, all the issues should be forensically examined and any lessons clearly identified and acted upon."
It added: "The final full independent report will be published as soon as possible, and all the agencies involved stand ready to take appropriate action."
Norman Lamb, who was the care minister in the coalition government, said the findings were shocking: "You end up with a sense that these lives are regarded somehow as slightly less important than others and there can be no second class citizens in our NHS.
"The thought is just horrifying and there have to be some answers from the trust."
- If you are directly affected by this issue, you can call this NHS number: 0300 003 0025.