The police watchdog asked Scotland Yard on Wednesday to hand over details of the case of the 15-year-old schoolgirl stabbed to death near Waterloo station in London to allow it to investigate a complaint that officers had failed to protect her.

The victim, Arsema Dawit, and her family told police at the end of April that she had been assaulted by a man who had also made death threats against her.

Police said the matter was still being investigated when Dawit was attacked in the lift of a block of flats in south London on Monday.

Officers found the schoolgirl suffering from multiple stab wounds, making her the 16th teenager to die violently in London this year. They have arrested a 21-year-old man in connection with the murder.

Media reports have said the suspect, who has not been named, could have been obsessed with his victim and been stalking her.

"The Independent Police Complaints Commission (IPCC) has asked the Metropolitan Police Service to refer the investigation into the handling of complaints from Arsema Dawit's family to it," the watchdog said in a statement.

"The family complained to the Metropolitan Police that Arsema was assaulted on April 30, five weeks before her murder. The IPCC awaits the referral, whereupon the Commission will assess how the investigation should be carried out."

Detectives say they are keeping an open mind about a motive for the attack but that it was not gang-related.

Media reports said the girl was still in her school uniform when a woman and her eight-year-old daughter found her with at least 10 stab wounds to her chest, neck and back.

She had lived with her parents and older twin siblings and had celebrated her birthday at the weekend, newspapers said.

Detective Chief Inspector Caroline Goode, who is leading the investigation, said in a statement: "At this stage I am trying to establish the true nature of the relationship between Arsema and the man in custody."