DNA tests may be carried out on men aged between 20 and 40 in the hunt for the killer of teenage model Sally Anne Bowman, police have said.
Model Sally Anne Bowman was killed just yards from her home
Miss Bowman, 18, was sexually assaulted and stabbed to death on 25 September.
Det Ch Insp Stuart Cundy said the serial sex attacker must live, work or have relatives in the South Croydon area, in south London.
He described her killer as a very dangerous man who must be taken off the streets to protect women.
A Met police spokeswoman told BBC News DNA tests on men under 40 was "always a possibility".
She said she was unable to confirm whether tests would focus on men living in the South Croydon area.
DCI Cundy added a potential witness, who helped a woman attacked an hour before by the man, is to be interviewed at length.
Sally Anne, who also worked as a hairdresser, was attacked just yards from her home in Blenheim Crescent in South Croydon at about 0415 BST.
Her partially-clothed body was discovered at about 0630 BST.
DCI Cundy said: "It's not a run-of-the-mill case. I believe he [the attacker] is a very dangerous individual.
"We need to arrest him and get him off the streets for the benefit of women who live in the area."
The latest witness came forward after an appeal by police on Monday.
The man went to the aid of a woman who was hit over the head with a metal object and robbed less than 500m away from where Miss Bowman was killed.
The man, in his early 20s, helped the woman in Sanderstead Road and apparently ran after the robber.
Forensic scientists have linked the murder to a sex attack in nearby Purley in 2001, but police said the attacker could be responsible for a further six sexual assaults.
Miss Bowman's possessions, a white handbag, mobile phone, cardigan and purse are still missing - as is the murder weapon, thought to be a 12cm to 14cm knife.
The attacker is thought to be white, in his 20s or 30s, between 5ft 9in and 6ft and of medium build with short brown hair.