BERLIN (Reuters) - German prosecutors said on Wednesday they were investigating letters telling voters that their political preferences had already been worked out from their internet postings and there was no need to go to the polls on Sunday. Some of the fake letters carried the logo of federal election authorities, meaning the sender could be guilty of forgery, said the state prosecutor s office in Wiesbaden. The messages were crude and  it would be surprising if people fell for it , a spokesman for the federal election director said.  Still, it s frustrating.  There was no indication of who was behind the letters, and they appeared to be based on a recent joke in a German magazine. But German government officials and security experts have been on the look out for evidence of meddling ahead of the Sept. 24 national vote, following allegations of Russian interference in polls in the United States and France. German computer magazine c t carried a satirical article this month in which a fictional voter is told that their vote will be automatically cast for a certain party given their behavior on the internet and credit card charges. The actual letters sent to German voters suggested the voters would cast their ballots for the  Party of Bible-abiding Christians  or the  Basic Wage Coalition,  RND newspapers reported. Prosecutors said the investigation started after a complaint from the German federal election director, whose office is in Wiesbaden.  The letters looked real, which is why we are investigating possible forgery of documents,  a spokesman for the prosecutor s office said. 