German police said Thursday they now doubt whether a chat room post attributed to a teenager who went on a shooting rampage is real.

Police spokesman Klaus Hinderer said authorities are looking into whether the Internet postings were faked. Hinderer's statement contradicted his earlier announcement that authorities were "completely convinced of the veracity of the post."

Click here for photos.

The post appeared in a chat room some six hours before the shooter, Tim Kretschmer, went on a rampage Wednesday at his former high school and killed 15 people in and around the southwest German town of Winnenden.

A joint statement released late Thursday by regional police and Stuttgart prosecutors said that, "in the course of the afternoon, doubts arose about the veracity of the Internet chat."

Hinderer said a search of Kretschmer's computer had shown no trace of his having made the chat room posting.

Stuttgart state prosecutors, who are leading the investigation, said they were trying to reach the U.S.-based provider of the site. A message posted to the site Thursday said, "No killing spree was announced here."

Prosecutors' Spokeswoman Claudia Krauth told The Associated Press that they shared the police doubts.

"It is diametrically contrary to our earlier announcement," Krauth said.

She would not provide details on why authorities now doubt the posts.

"Now we need to check whether it's really false, how that could have been done and who it was," Krauth said.

Police said they learned of the chat from a Bavarian teen who told his father and then police about it when he realized the threat had been real.