DNA screening of about 4,000 men is due to begin as police hunt for the killer of teenage model Sally Anne Bowman.
Sally Anne Bowman, 18, was sexually assaulted and stabbed to death yards from her home in South Croydon, south London, last September.
Police believe her killer has links to the area and they have been writing to homes and workplaces asking men to volunteer for the test.
The mass screening starts at 0700 GMT at the Brighton Road Baptist Church.
Voluntary process
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.
"None of those DNA samples or fingerprints will be used to check out other unsolved crimes"
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 fingerprints will be used to check out 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."
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.
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