Police have released an image of a man suspected of killing teenage model Sally Anne Bowman.
The suspect is white, in his 20s or 30s, with short brown hair
Miss Bowman, 18, was sexually assaulted and stabbed to death just yards from her south London home on 25 September.
Police said DNA tests may be carried out on men under 40. It is thought the killer lives or works locally.
They have also offered a £20,000 reward for information leading to a conviction and will make a televised appeal on BBC One's Crimewatch on Wednesday.
A new photograph has also been released of Miss Bowman on holiday dressed in the clothes she was wearing on the day of her death.
Miss Bowman was dressed in these clothes on the day she died
Det Ch Insp Stuart Cundy believes the man they are looking for is a serial sex attacker who may have committed around six other assaults.
Miss Bowman's murder in Blenheim Crescent, South Croydon, has been forensically linked to a sex attack in nearby Purley in 2001.
It is thought the same man attacked and robbed another woman just hours before killing the model.
A potential witness, who helped a woman attacked an hour before by the man, is to be interviewed at length, he added.
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.
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 described as white, in his 20s or 30s, between 5ft 9in and 6ft and of medium build with short brown hair.
Announcing the reward on Wednesday, DCI Cundy said: "Every piece of information, no matter how apparently insignificant, could provide vital evidence or a new point for further investigation."