About 4,000 men working and living in South Croydon are being asked to voluntarily give their DNA as part of the hunt for a teenage model's killer.
Sally Anne Bowman was sexually assaulted and stabbed
Sally Anne Bowman, 18, was sexually assaulted and stabbed to death just yards from her home in south London, in September last year.
The attack has been linked to a sexual assault on a woman in nearby Purley.
Police are sending letters to workplaces and homes in the area asking men to come forward.
Officers believe the attacker has a good knowledge of and strong links to the area.
Det Ch Insp Stuart Cundy, who is leading the murder investigation, said the DNA profiles would not be put onto the national database and would only be used to eliminate men from the inquiry.
He said: "What I'm appealing for is men from the South Croydon area, men who live there, work there or visit there, who were born between 1965 and 1985, that's about 20 to 40-years-old, and are either white or light skinned, because that's the description of the man I believe attacked Sally Anne Bowman.
"It is an entirely voluntary process. None of those DNA samples or finger prints will be used to check out an other unsolved crimes.
"Obviously if someone does refuse then each case will be reviewed on its own merits.
"I'll look at the evidence available, how their name entered this murder inquiry and obviously we'll give careful consideration to whether someone should be arrested for Sally Anne's murder."
A temporary police office has been set up at the Brighton Road Baptist Church.
Men who receive the letters are asked to drop in. They must not have eaten, drunk or smoked in the 20 minutes before their visit and will need to take two forms of identification.