By Daniel Emery
Technology reporter, BBC News
Victims are trying to understand why the massacre happened
In the aftermath of the fatal school shooting in Germany - where 17-year-old Tim Kretschmer murdered 15 people before turning the gun on himself - police said Kretschmer had posted a warning of his attack on the internet.
Less than 24 hours after the attack, Wurttemberg's Interior Minister - Heribert Rech - had read out a message thought to have been posted by Kretschmer.
"I have weapons here, and I will go to my former school tomorrow and then I will really do a grilling
you will hear of me tomorrow. Remember the place's name: Winnenden."
But it is now looking probable that the internet posting was a hoax.
"Doubts emerged during the afternoon about the veracity of the entry in the internet chatroom. Of course every piece of information, especially concerning this entry, is being vigorously examined," Waiblingen police said in a statement.
The German internet site - www.krautchan.net - on which the message was alleged to have been posted, has been temporarily shut down.
On its homepage, a message apparently from the website owners says: "No killing spree was announced here".
Tim Kretschmer was treated for depression in 2008
"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," it contested.
So was it fake and if so, how? And if it was, why were the police and the world's media taken in?
Speaking to the BBC, Thomas Maile - a spokesman for Waiblingen Police - said the tip-off arrived via e-mail.
"A police officer got two e-mail messages, I don't know if they were URL links or an attachment, that's something we are checking now.
Either way, the apparent warning was traced back to Kratchan.net. This is an image hosting site, rather than a pure chatroom, which means users can post images, as well as text.
According to the owners of the site, no such warning was ever posted. They say they have trawled their site and have found posts that could be from Kretschmer, with the same time and date stamp as the apparent warning, but that the purported warning itself does not exist on their server.
"No killing spree was announced here, there are only people who know how to use Photoshop," the site said on a statement.
That statement holds a clue as to how a warning could have been faked.
The alleged internet warning
Krautchan.net said this was the only relevant post they could find on their site
One way would be to use image manipulation software - such as Photoshop or Paint Shop Pro - to create a fake message that would look similar to the original, but with different content.
Because Krautchan.net is an image hosting site - rather than a true chatroom - it would be very easy for someone then to upload that doctored image.
Of course, the warning would have appeared as an image, rather than text embedded in the site, which should have set alarm bells ringing - except that, as soon as news of the warning came out, the website collapsed under the sheer number of people trying to log onto the site.
Once that happened, it was impossible to check the veracity of the warning; the only thing the world's media had to go on was the word of German authorities.
"At the time we had no contact with the website, which was based in the US. Now we are in contact with them and have support from police in the US. We have written to them and now we will wait to see what they have to say," said Mr Maile.
It is very easy to see why the German authorities thought the warning was real.
Eric Harris and Dylan Klebold - who murdered 12 people at Columbine High School on 20 April 1999 - posted violent threats directed at students and teachers at the school on their own website.
Cho Seung-hui, who murdered 32 people in a three-hour rampage at Virginia Tech, submitted essays as part of his creative writing course that were so disturbed, his professor referred him to counselling.
He subsequently created a long, ranting video, to be broadcast after his killing spree, in which he described his fellow students as "brats", "snobs" and "rich kids", who he said had "raped his soul" and forced him to do what he did.
The posting on Columbine killer Eric Harris's website gave an insight into his plans
A US study in 2001, which examined 37 separate school shooting incidents, determined that none of them were impulsive actions.
"It was not a case of a kid getting up in the morning and saying, 'I'm going to take a gun to school today and shoot somebody','' said Bryan Vossekuil, co-director of the Secret Service's Safe School Initiative, which conducted the study.
Centre of attention
Looking at the image that contained the apparent warning and the post that Krautchan.net claims are the posts they found on the site, one can see how easy it would be to change one into the other.
"The message we received could be fake. But the postings [on Krautchan.net] could also be fake. All pictures on the internet can be fake. We need to speak to the host and find out what is real and what isn't," said Mr Maile.
So if the message was faked, why would someone do it, especially on a story that has devastated so many lives and shocked people across the globe?
Speaking to the BBC, Professor Andrew Silk - a criminal psychologist at the University of East London - said in most cases, it was attention seeking.
"In some ways it's similar to people claiming responsibility for crimes they didn't commit.
Humble was jailed in 2006 for misleading the Ripper investigation
"Some are delusional, with mental health issues, while others are seeking attention and trying to create a stir.
"They are seeing the world's media get whipped up and they know they caused it, even though they [the person creating the hoax] know they are anonymous. They feel linked to the story and that's what drives then," he said.
There have been other infamous hoaxes in the past. In 2006 John Humble was sentenced to eight years for perverting the course of justice over the Yorkshire Ripper killings.
His admission brought an end to the "Wearside Jack" mystery - the nickname given to the caller behind the infamous bogus tapes which claimed to have been made by the killer.
Thomas Maile said police would take a dim view on the author, if the internet warning proved to be a hoax.
"If this is a fake, and we know who that person [who created the internet post] is, then we would want to speak with them."