A 16-year-old girl seriously injured by a classmate who then leapt to his death on Wednesday sent an instant message asking for help from her boyfriend before the attack.
But she was unable to answer the phone when her boyfriend called back, police say.
The picture emerged yesterday of the desperate moments before Cheung Pui-ling suffered multiple wounds that left her in hospital. She was in a stable condition last night but was still not fit for a full police interview.
The attack was discovered at about 2pm on Wednesday after Lin Yiu-wai, also 16, leapt to his death from a 20th-floor flat at Man Wo House in Wo Che Estate, Sha Tin.
Officers believe Cheung retreated to the kitchen of Lin's home after he suddenly started saying strange things as the two teenagers revised together. She used her mobile phone to send the message for help through her Facebook account, a police source said. "But she was unable to answer the call when her boyfriend telephoned her," the source said.
Officers found the injured girl semi-conscious in the living room of the flat.
She was able to give police a brief account of the attack before passing out, the source said.
"She said she was attacked with an unknown hard object and [Lin] tried to suffocate her by squeezing her neck and hitting her head against the floor."
Cheung had lacerations to her neck, the back of her head, her lips and her arms but police remained unsure yesterday what weapon was used.
A meat cleaver and a pair of scissors were found in the kitchen sink but there was no blood on them. There were broken glass fragments in the flat.
Cheung was admitted to the nearby Prince of Wales Hospital in serious condition on Wednesday night but her condition after surgery was stable.
"Her condition is not life threatening," the source said.
Detectives from Sha Tin are investigating what triggered the attack. One possibility was that Lin might have been unhappy that a former girlfriend was about to leave to study in Britain.
The source said Lin, described as an elite student, had not showed any previous abnormal behaviour and had no history of mental illness.
"We were told that he was also a good boy at home," he said.
Cheung's boyfriend is also a schoolmate and a good friend of Lin. All were Form Four students at Mrs Fung Wong Fung Ting College in Sha Tin.