The hunt for a suspected pensioner serial killer in Germany was called off on Wednesday after police said they had been fooled by a prankster suffering from mental health problems.

Police began receiving a few years ago letters by someone admitting to the 1962 slaying of schoolgirl Lydia Schuermann and the 1970 killing of prostitute Heiderose Berchner as well as 11 other murders between 1961 and 1992.

Investigators took them to be genuine and believed they were closing in on the murderer when earlier this month they began carrying out DNA tests on more than 5,000 pensioners in a small town near the French border.

But they were alerted to the fact they might be on a wild goose chase when a postman said he recognised the writing in postcards released by police in their appeals for help from the public.

Police then located the 34-year-old prankster who admitted to writing all the letters and in his flat they found recordings of police programmes on unexplained crimes dating back to the 1960s.

`(This) obviously explains his detailed knowledge of the crimes,` police in the state of North Rhine-Westphalia said in a statement. `He fully admits to writing all the letters.`

The man has been living in a psychiatric clinic since the beginning of November and doctors believe he wrote the letters in order to draw attention to himself, doctors said.