The man accused of beheading a British woman in Tenerife would normally have had follow-up care after treatment in north Wales, says a psychiatrist.
Patrick Purcell says psychiatric patients detained for their safety are given community care unless they leave the area.
Deyan Deyanov was detained for his own safety at Glan Clwyd Hospital's Ablett Unit in Denbighshire, last summer.
Betsi Cadwaladr University Health Board is now reviewing its records.
Health officials have not said whether Mr Deyanov received follow-up care.
Dr Purcell, who specialises in assessing people under the Mental Health Act, said, generally speaking, follow-up care was an "imperative part of discharge planning" as it was "a very vulnerable time in all psychiatric illnesses".
"In certain cases there are actually legal elements called [a] community treatment order but the vast majority of patients would be supervised very closely because we know that people who have just been discharged are highly likely to be readmitted," he told BBC Radio Wales.
When asked if it was possible for people to be discharged into the community without any further care, he said: "That would be very unlikely unless the person decided not to have contact or if they left the area."
On Thursday the health board which manages the hospital in Bodelwyddan said it had provided assistance to North Wales Police "in relation to a person detained for his own safety in the summer of 2010" but that it had "no further contact with this client since October last year".
A statement said: "We are currently reviewing all records relating to the case in line with standard clinical procedures.
"We will be co-operating with other statutory agencies but have a duty to protect patient confidentiality and will therefore not be making further statements regarding the case at this time."
It is not clear if Mr Deyanov, 28, was allowed to leave the psychiatric unit after being assessed as safe, or if he absconded, and whether a follow-up care plan was drawn up.
It is being reported he was sectioned under the Mental Health Act.
Last week, Jennifer Mills-Westley, 60, was murdered in Los Cristianos.
Mrs Mills-Westley, originally from Norwich, Norfolk, was inside a supermarket on Avenida Juan Carlos when she was stabbed and decapitated.
The grandmother of five had been living in Tenerife after retiring from her job as a road safety officer with Norfolk County Council.
Dr Purcell explained that patients could be detained in hospital under the Mental Health Act for up to 28 days - a Section 2 order - or up to six months under a Section 3.
He said under a Section 2, two doctors and a social worker assessed whether a patient should be detained in the "interest of their health which is the most common ground, or the interest of their safety or the interest of protecting others".
He said the most common reason for being held for up to six months was usually for a patient's own safety.
Reports from Tenerife suggest Mr Deyanov had previously received psychiatric treatment on the island.
A search and arrest order was issued for Mr Deyanov on 10 May, three days before the killing.
The BBC's Sarah Rainsford said a magistrate had ordered his arrest in connection with a violent attack.
Mr Deyanov was arrested on suspicion of murder and is now being held at a psychiatric unit in Tenerife.