German police began taking DNA samples from around 5,000 pensioners in a small town on Thursday as they investigated two murders committed four decades ago.

Police had long since given up on solving the 1962 murder of 13-year-old Lydia Schuermann and the 1970 killing of 29-year-old prostitute Heiderose Berchner but a few years ago anonymous letters began appearing.

Police found a DNA match on the letters and decided to centre their investigation on Weiskirchen after the mayor of the small southwestern town received another anonymous letter containing an advert from a local newspaper.

Police said Thursday that the latest letter they had received said that the writer did not live in Weiskirchen but investigators pressed ahead with the voluntary tests in case this was an attempt to put them off the scent.