A depressed woman facing divorce strangled her 10-year-old son before hanging herself in their Tai Koo Shing flat, police said.
The tragedy happened after Lui Miu-man, 47, was released last week from Pamela Youde Nethersole Eastern Hospital's psychiatry department in Chai Wan.
Police believe Lui strangled her only child, a Primary Five student at North Point Methodist Primary School, between midnight and yesterday morning.
Lui's husband and the boy's father, Yu Yan, 40, discovered the bodies when he returned home at Banyan Mansion at 9.20am.
Police said the boy was found lying on the bed with marks to his neck, while Lui was found hanged in the bedroom.
A suicide note was found. Postmortems will be conducted tomorrow. The case is classified as murder and suicide.
Chief inspector (Eastern District) Chow Siu-tong said Lui had attempted suicide by burning charcoal in September and her case was followed up by the Social Welfare Department.
Chow said the family had no record of domestic violence, but a source indicated an extramarital affair and the woman was in the midst of a divorce with her husband, an instructor at the University of Hong Kong's School of Chinese.
Pamela Youde Hospital confirmed that Lui once received psychiatric treatment.
Doctors advised her to stay in the hospital for observation but her relatives had her discharged last week.
The Social Welfare Department said social workers received and followed up Lui's case at the end of last year.>North Point Methodist Primary School expressed condolences and said teachers of its risk management team will explain the incident to students today. The Education Bureau said consellors and an educational psychologist are deployed at the school.
Against Child Abuse director Jessica Ho Oi-chu said: "Children deserve the right to live and no one can deprive them of that."
When dealing with mentally ill relatives, Ho advised family and friends to stay highly alert, particularly if the patients had previously tried to harm themselves.
"It helps to send the children away for temporary care," she added.