Hospitals investigate just one in seven deaths of vulnerable patients in NHS care, according to new figures.
Mental health trusts in England probed only 209 of 1,436 deaths of patients with learning difficulties on wards since 2011, data obtained by the Guardian through Freedom of Information laws suggests.
And just 100 (36.2%) of 276 unexpected deaths in the same period were investigated, prompting concern from the health watchdog and campaigners.
It comes after Health Secretary Jeremy Hunt ordered a review last weekinto how deaths are investigated by NHS trusts following a report into Southern Health, which found the mental health trust's serious incident requiring investigation (Siri) processes were "inadequate".
The latest figures, from 47 trusts, also showed only five hospitals had investigated all deaths of patients with learning difficulties under their care, while three trusts - Somerset Partnership, Northamptonshire Healthcare and Rotherham, Doncaster and South Humber - probed none of the 146, 63 and 28 such deaths on their wards respectively.
And of the 21 trusts which recorded unexpected deaths, 11 launched probes into fewer than half.
Professor Mike Richards, England's chief inspector of hospitals, told the Guardian: "The findings from this investigation are very concerning.
"We're keen to work with the Guardian to look at the new information in more detail. This will help us to plan the review that the Care Quality Commission (CQC) is already committed to doing."
The probe into Southern Health showed the trust failed to look into the deaths of hundreds of people since 2011 and campaigners warned the problems were more widespread.
Jan Tregelles, chief executive of Mencap, said the failings at Southern Health were systematic of issues across the NHS.
She told the paper the figures will "show the need for the Government to commission an independent investigation across the NHS on these failings".