A father was taken to hospital by police weeks before he was found dead with his sister-in-law and two children.
Bartlomiej Kuczynski, 45, was found at a house in Costessey, near Norwich, on Friday, along with Jasmin Kuczynska, 12, Natasha Kuczynska, eight and Kanticha Sukpengpanao, 36,
All died from stab wounds to the neck.
Mr Kuczynski's death is not being treated as suspicious but the other three are being treated as murder.
The four bodies were discovered by police officers at a house in Allan Bedford Crescent, Queen's Hills, on Friday morning.
It has emerged that Mr Kuczynski was taken to the emergency department at Norfolk and Norwich University Hospital by police on 14 December, the same day he was reported as a missing person.
He left the unit after being assessed as having "mental capacity", the hospital said.
A spokesperson for the hospital said: "We can confirm that the patient was taken to the emergency department by police on December 14.
"Following a comprehensive clinical assessment, the patient was assessed as having mental capacity and he left the unit before being reviewed again by the team."
The fact Bartlomiej Kuczynski was voluntarily taken to A&E because of his mental health in December proves he did have prior contact with health services.
By Nikki Fox, BBC health correspondent, East of England
Police took Bartlomiej Kuczynski to A&E after they sought telephone advice from a mental health team about his behaviour, I understand.
The team told police to get him checked over, so officers took him to A&E and left him there.
He had a clinical assessment - but that does not mean he had a thorough mental health assessment or a follow-up with a mental health professional. We don't know how long he waited in A&E but he left before this was carried out.
Even though he was judged as having capacity, it's not known if concerns were raised by the mental health liaison team at the hospital when he left.
Nor is it known if concerns had previously been raised by Mr Kuczynski's family regarding his state of mind, either to the police or mental health services.
Norfolk Police has referred itself to the Independent Office for Police Conduct (IOPC) over the case.
On Tuesday, the IOPC confirmed Mr Kuczynski called 999 shortly before 06:00 GMT from inside the house.
He said he was confused and worried about his mental state.
It is understood he was advised to seek medical advice and contact the NHS on 111.
Police were not despatched until after they received a second 999 call at 07:00 GMT from a member of the public and forced their way into the house