More than a dozen threats of violence have been made against German schools since a teenager went on a deadly rampage in the southwewstern town of Winnenden two days ago, officials said today.

Most of the threats were quickly dismissed as frivolous but at least two schools have been evacuated and a 20-year-old was arrested in Esslingen, near Winnenden, after threatening on the internet to go on a rampage.

But police appear to be treading carefully after announcing yesterday that Tim Kretschmer, the 17-year-old who killed 15 people after launching a shooting spree at his former high school in Winnenden, had warned of the attack in a chatroom message hours before.

In what was presented as a breakthrough for investigators, Heribert Rech, the state's Interior Minister, told reporters that Kretschmer had left a message on the site Krautchan.net at 2.45am on Wednesday - almost seven hours before the attack.

In it, Kretschmer complained of his "sh**ty life" and said that he was going to shoot up his old school because "everyone is laughing at me".

But, as The Times reported yesterday, that message never existed except in a fake image file first posted on the website three hours after Kretschmer committed suicide. A Krautchan moderator, Tsaryu, said: "I even know who posted it, and he's from Düsseldorf. That's not even near Baden-Württemberg."

Mr Rech now admits it was a fake. "Some crazy person obviously put out this dreadful false message," he told the Süddeutsche Zeitung newspaper. "It must have been made up afterwards."

The spate of copycat threats also spread beyond Germany's borders. A Swedish teenager was arrested yesterday and Dutch police today picked up an 18-year-old said to have made another online threat against schools in the southern city of Breda. A police spokesman said that the suspect had insisted that it was a joke.

This morning, police closed a school in Ilsfeld, some 20 miles (30km) northwest of Winnenden, after discovering a threat against it on an online chatroom at 3am.

The school was cordoned off first thing and its 650 pupils sent to a nearby sports hall but sniffer dogs found nothing suspicous during a search of the building. "We are taking the threat seriously," Roberto Monaci, a police spokesman, said.

Meanwhile, in the town of Wendlingen, where Kretschmer killed himself after being wounded in a shootout with police, officers were called in after a 15-year-old scrawled the words "shooting spree" in chalk in his schoolyard.

One official said that half a dozen hoax calls were made to schools in Baden-Württemberg yesterday alone, although none was considered credible enough for the schools to be evacuated.

It emerged yesterday that Kretschmer, whose victims included nine pupils at the Albertville technical school, eight of them girls, also shot dead a man at a psychiatric clinic where he had been treated for depression,.

Kretschmer was supposed to attend appointments at the clinic in Winnenden but broke off the treatment. Minutes after fleeing the school on Wednesday morning, he killed a man, thought to be a gardener, outside the clinic.

He then commandeered a car, held a gun to the head of his 41-year old hostage and asked him: "Should I have fun and pick off some more drivers?" He went on to kill two more people at a car showroom in the nearby town of Wendlingen before committing suicide.

With the internet message shown to be a hoax, investigators are still struggling to understand the motives of a teenager whose obsession with fictional violence might have contributed to the all-too-real blood-bath that has shattered the normally quiet town in which he grew up.

Detectives who seized Kretschmer’s home computer discovered violent video games and pornography. Friends said that he regularly played the computer game Counter Strike and had become increasingly fascinated with horror films. His parents insisted, however, that Kretschmer also liked comedies and action films.

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