An online warning which police say was posted by a German school killer was dismissed last night as a crude internet fake.

German officials said today that Tim Kretschmer, 17, left a message on a chatroom early Wednesday morning in which he promised to shoot up his old school in Winnenden because "everyone is laughing at me".

According to the police account, Kretschmer left the message on the Krautchan.net site at 2.45am, almost seven hours before the shooting spree, in which he killed 15 people before turning his gun on himself.

The posting was said to have provided the first clear evidence as to the gunman's motives - he wanted to take revenge because nobody at his former school took him seriously - and provided a clear link with similar school shootings in the past.

The message in question included the lines: "Damn it Bernd, I've had it. I'm sick of this shi**y life, always the same — everyone laughing at me, nobody sees my potential. I'm serious Bernd — I got guns here, and I'm gonna head over to my old school tomorrow and have myself a good ole barbecue."

Heribert Rech, interior minister of the state of Baden-Württemberg, told a press conference that the father of a teenager from Bavaria had contacted police to tell them about the warning.

But Krautchan.net, and its users, immediately denied that any such warning was ever posted on the site and The Times has been told that the message in question was in fact a faked-up image file first posted on Krautchan more than three hours after Kretschmer had committed suicide.

A moderator on the site, identified only as Tsaryu, told The Times: "I even know who posted it, and he's from Dusseldorf. That's not even near Baden-Wuerttemberg."

Krautchan is what is known as an "imageboard" site or "Chan" - which means that its primary purpose is for the posting and discussion of images, many of them ludicrous photoshopped images of a topical nature.

According to a screen-capture of the discussion thread in question, the post containing the image of Kretschmer's message was posted on the site at 4.57pm with the message: "Look at the timestamp on this." Kretschmer had died at 12.30pm.

The "fake" message was timestamped at 2.45am and carried the ID code 485465, both the same as a post 14 hours earlier which started off the online conversation.

Since no two posts can share the same ID, other users immediately cried foul. "Photoshop," the next user wrote.

It is not clear whether the police had seen the entire thread before yesterday's press conference or had simply been sent a copy of the image.

"As one of the moderators of Krautchan I can assure you that the screenshot in question is indeed forgery," Tsaryu said in an e-mail to The Times.

"The faked screencapture (No.486231) was uploaded AFTER the killing spree."

The moderator also pointed out that German media reports had spoken of a conversation between Kretschmer and a friend called Bernd. In fact, under Krautnet protocol, all communications are anonymous - and all users adopt the name Bernd.

"The user in question, who reported it to the police was indeed from Bavaria, but neither was it his friend, nor was he named Bernd. It was one of the many anonymous users on Krautchan."

The homepage of the Krautchan.net site today carried a curt message, in both German and English, announcing that it was down because its servers could not cope with the rush of traffic.

It added: "There's nothing to see anyway, since the German press was unfortunately fooled (and probably not for the first time either) by a forgery.

"No killing spree was announced here, there are only people who know how to use Photoshop."

After insisting throughout the day yesterday that the posting was genuine, police conceded last night that they had doubts about its authenticity.

Klaus Hinderer, a police spokesman, said a search of Kretschmer’s computer had shown no trace of his having made the posting in a chat room conversation.

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