The family of murdered schoolgirl Arsema Dawit have demanded to know why police failed to protect her from a stalker who threatened to kill her. The 15-year-old was stabbed up to 10 times in a frenzied attack in the lift of the block of flats where she lived with her family in south London. Her family told police just weeks ago that Arsema had been assaulted by an obsessive man. But they said officers told them there was nothing they could do about the man who met her at an Eritrean church where she was a member of the choir. Scotland Yard confirmed that police received a complaint on April 30 that a man aged 29 or 30 had assaulted Arsema in a McDonald's restaurant in south London on April 16 and made threats to kill her. An officer spoke to the girl at her school on May 12 but she claimed to have "no knowledge" of the incident. Police contacted Arsema's mother a week later and the investigation was still ongoing when she was murdered on Monday. One source close to the inquiry said officers were not aware of a wider campaign of harassment against the schoolgirl. A 21-year-old man remained in custody after he was arrested on a footbridge over the River Thames as he threatened to kill himself, police said. Arsema was the 16th teenager to be killed in the capital this year and the first female victim. The attack came days after Scotland Yard Commissioner Sir Ian Blair vowed to do everything in his power to combat knife crime. Arsema's body - still clothed in her pink and blue school uniform - was found slumped in a lift at Matheson Lang Gardens on Monday afternoon. The girl, who celebrated her 15th birthday three days ago, had been subjected to a horrific attack and left for dead as she walked home from school.