The family of a schoolgirl stabbed to death in south London had complained of her being attacked before, police said.
The body of Arsema Dawit, 15, was found in a lift at Matheson Lang House, Baylis Road, Waterloo, on Monday.
A post-mortem examination showed that the south London teenager died from multiple stab wounds.
Neighbours and friends said Arsema was being "harassed" by a man and her family had contacted police about it. A 21-year-old man has been arrested.
Police said an allegation of assault and threats to kill against Arsema were reported in April. They added that she had known the man held for her killing.
The man was arrested nearby on Monday and is being questioned by detectives over the killing.
Arsema, who had turned 15 on Friday, is believed to have returned home to the block of flats where she lived when she was attacked.
'Threatened to kill'
Paramedics arrived at the scene within four minutes but were unable to revive the teenager, who suffered multiple wounds, a London Ambulance Service spokesman said.
Det Ch Insp Caroline Goode said: "At this stage I am trying to establish the true nature of the relationship between Arsema and the man in custody.
"We are speaking to her family and friends to help build a picture of events leading up to the murder and to help piece together the relationship between them."
A Metropolitan Police statement said an allegation of assault and threats to kill against Arsema were made on 16 April in a McDonald's restaurant.
The police investigation is being stepped up
They said she "was unaware of the incident and had no knowledge of it" when a safer schools officer spoke to her about it on 12 May.
"Despite this we continued to investigate the circumstances of the incident and Arsema's mother was subsequently contacted on the 19 May," the statement added.
"The investigation was still being progressed when Arsema was tragically murdered yesterday."
Neighbour Wayne Fort, whose partner and eight-year-old daughter found Arsema in the lift, said: "There was a chap who seemed to be infatuated with her. He seemed to have met her at the church.
"I could see from the efforts of the elders of the family they were trying to get rid of the man."
Arsema, an Eritrean national, is believed to be the 16th teenager to have died violently in London since the start of the year.
Police officers have distributed leaflets on the estate in a bid to reassure people and telling them the killing is not gang-related or a stranger murder.