Police on Saturday said finding out the motive for brutal knife attack that killed two women at a busy Diamond Hill shopping mall would be a top priority, adding that there was nothing to suggest that their prime suspect, a jobless 39-year-old man with a history of mental illness, knew his victims.

At a media briefing in the early hours of Saturday, Senior Superintendent Chung Lai-yee of the Kowloon East Regional Headquarters said the man first stabbed one of the women repeatedly with the knife, before going on to attack her friend who tried, unsuccessfully, to intervene.

She said officers arrived at the scene soon after to find the two victims – aged 22 and 26 – lying unconscious on the ground with multiple knife injuries.

They were pronounced dead at United Christian Hospital after suffering massive bleeding. Officers subdued and arrested the 39-year-old man at the mall.

"The motive of this suspect is one of our most important police investigations," Senior Inspector Hui Ham-fong told reporters at the briefing. "We will endeavour all our efforts to bring the suspect to justice."

On the question of whether the killer knew his victims, she added: "There is no concrete evidence to support that the suspect has any acquaintance with the two victims."

The attacker is said to have entered the shopping mall 15 minutes beforehand and bought a 12-inch knife from a shop, which he subsequently used to slash the victims.

The force appealed to people who might have witnessed the brutal incident to come forward with information.

The suspect was being held for questioning overnight.

Earlier, the Social Welfare Department advised people not to share the video of the attack, which has been circulating online. It also urged those who experience emotional distress after watching the video to seek professional help.

The department said people can call its 24-hour hotline on 2343-2255 for immediate telephone counselling, support and referral services. They may also call the hotline of the Hong Kong Red Cross on 5164-5040 for assistance.