Staff at a mental health hospital said they had not felt safe before a patient was killed by another patient.
Robert McNeill, 60, died at Great Oaks Hospital in Scunthorpe after being attacked by fellow patient Jamie Reed in January 2017.
An NHS England report said "risks were not adequately controlled and caused staff to be anxious about their own and patients' safety".
The hospital trust said it had made improvements to "the ward environment".
Reed, 26, was detained for 12 years in a secure unit after he was convicted of manslaughter by diminished responsibility following a trial at Hull Crown Court in July 2017.
The court heard Mr McNeill was left with multiple injuries to his face, head, back and neck after a "frenzied assault" by Reed.
The independent NHS England report into the attack said the assault on Mr McNeill could not have been predicted or prevented by the trust.
However, it added that "staffing levels and skill mix on the ward had a significant impact on the conditions in place at the time of the incident".
"Staff reported that they did not feel safe, particularly on nights and on the part of the ward where they had raised concerns about staffing".
Dr Nav Ahluwalia, from Rotherham Doncaster and South Humber NHS Foundation Trust (RDaSH) apologised to the men's families for not offering a "better standard of care and treatment" and said that "patient safety is paramount to us".
In a statement, RDaSH said: "We are sorry that staff felt anxious.
"We have worked incredibly hard with our staff on this unit, recruited more staff, and have worked hard to improve the ward environment, culture and the care we offer."