It was later reported Harrison had been assessed for mental health issues at the Royal Edinburgh Hospital earlier in the same day.
His partner, Cheryl Yorkston, is believed to be recovering with family after being treated for a serious stomach wound.
“He had been to the doctor about his problems and was trying to get help,” a neighbour of Cheryl’s was quoted as saying. “It sounds like he might have fallen through the cracks. It look like he wasn’t in any condition to be out.”
Worried neighbours are understood to have called police after terrified Cheryl went running into the street screaming for help. Officers arrived to find Harrison covered in blood.
Detectives cordoned off Cheryl’s home and closed Main Street for three hours while they launched an attempted murder investigation.
Jobless Harrison is understood to have received treatment for mental health issues, while a police source has been quoted as claiming he had a history of problems. The source said he was known to officers for a variety of reasons, including drugs, disorder and sex offences.
It was reported Harrison was registered as a sex offender in 2008 but no longer signs the register or is subject to restrictions. He is understood to have been a patient at Herdmanflat Hospital, an NHS mental health centre in Haddington.
A spokeswoman for NHS Lothian refused to comment on an individual case.
“Harrison was always nice and friendly when he was around,” added the neighbour.
“He spoke about problems he had. He said he attended Herdmanflat Hospital but never said exactly why. You would see him at her house, then you wouldn’t see him for weeks or months.”
Harrison, originally from Edinburgh but living in Haddington, is thought to have been in a relationship with Cheryl for three years.
His neighbours described him as “odd” and said loud music was often heard coming from his home late at night.
A Police Scotland spokeswoman said: “Police in East Lothian are investigating following an attempted murder at an address in Longniddry. The incident happened at about 9.15pm on Friday, June 23. A 34-year-old man sustained serious injuries and remains in a critical but stable condition in hospital.
“A 51-year-old woman also sustained injuries and has been subsequently discharged from hospital.
“Inquiries are currently ongoing. A report will be submitted to the Procurator Fiscal.”