Tim Kretschmer fired more than 100 shots on Wednesday, police say
German police say an internet warning which appeared to be from a youth who later carried out a deadly firearms attack on a school may have been fake.
The message was originally believed to have been posted by Tim Kretschmer six hours before he killed 15 people in the towns of Winnenden and Wendlingen.
But police later told the BBC they had been contacted by internet users and officials who doubted its origins.
They have filed a request for access to the website's US-based server.
Local police spokesman Klaus Hinderer told the BBC they had been alerted to a possible problem with the message by internet users and official sources on Thursday afternoon.
Regional police and Stuttgart prosecutors later released a statement confirming those concerns, saying that "doubts arose about the veracity of the internet chat".
The internet warning is revealed
Investigators said it appeared that Kretschmer's computer had not been used to send the message in question.
The German internet site on which the message was alleged to have been posted has been temporarily shut down.
It has been replaced by a message apparently from the website hosts saying: "No killing spree was announced here.
"We don't know what exactly the authorities claim to have found on the perpetrator's PC. Maybe he visited the site, but he definitely didn't write the post that went through the news, because that one never existed."
As the website's servers are based in the US, German police have filed an official request through Interpol to obtain permission to recover information stored on them.
Kretschmer killed 12 people in the school and three in the nearby town of Wendlingen before taking his own life.
He shot himself after a shoot-out with police in a nearby town.
At a news conference earlier on Thursday, Baden-Wurttemberg's Interior Minister Heribert Rech had read out the message purportedly posted by Kretschmer.
"I've had enough. I'm fed up with this horrid life... Always the same," it read.
"People are laughing at me... No-one sees my potential... I am scared, I have weapons here, and I will go to my former school tomorrow and then I will really do a grilling."
The message then continued: "Possibly I get away, so keep your ears open, you will hear from me tomorrow. Just remember the name of the place, Winnenden."
Mr Rech said a German man had alerted police about the internet warning after the school shooting.
The man said his teenage son had told him about the warning only after seeing the news reports. He had not previously taken the threat seriously, responding to Kretschmer's post with "LOL", the chatroom shorthand for "laugh out loud".
Flags were flying at half-mast across Germany on Thursday as a mark of respect for the victims of the shootings.
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