The death of a teenager being cared for at an Oxfordshire in-patient unit for people with learning difficulties was "preventable", a report has concluded.
Connor Sparrowhawk, 18, was found unconscious in the bath at the unit in Slade House, Headington, on 4 July and died the same day in hospital.
The independent Verita report, published on the trust's website, found it had "failed significantly" in his care and treatment.
The trust has issued an apology.
The report follows a scathing review of the assessment and treatment unit by the Care Quality Commission (CQC), published in November, which found it failed on a number of standards, including patients feeling unsafe.
The unit has been closed to new admissions since the CQC report.
Connor had epilepsy and experienced seizures.
A post-mortem examination showed he died as a result of drowning, likely to have been caused by a seizure.
The report said the failure of staff to respond to and risk-assess Connor's epilepsy led to a "series of poor decisions around his care".
It added there had been no "comprehensive care plan" in place to manage his epilepsy, which was also not considered as part of his risk assessment - breaching National Institute for Health and Care Excellence (Nice) epilepsy guidelines.
The report also found the 15-minute observations in place at Connor's bath times were "unsafe".
It said the unit "lacked effective clinical leadership" and described the team working there as "weak".
Katrina Percy, chief executive of the Southern Health NHS Foundation Trust, said she was "deeply sorry" it had "failed to undertake the necessary actions required to keep him safe".
"We are wholly committed to learning from this tragedy in order to prevent it from happening again and I would like to apologise unreservedly to Connor's family," she added.
She said the trust would work to address the findings and recommendations of Verita's report and said a number of actions had now been taken, including reviewing staff training in relation to the care and risk assessments of patients with epilepsy.
In a statement issued by Bindmans' solicitors, on behalf of Connor's family, his mother Sara Ryan encouraged people to read the report and said "remember that Southern Health were quick to write Connor's death off as natural causes".
"He should never have died and the appalling inadequacy of the care he received should not be possible in the NHS," she added.