Winnenden, Germany - The German teenager who shot dead 15 people in a school massacre disclosed his intentions on the internet hours beforehand, officials said Thursday.

At a press conference, police and other officials painted a picture of a disturbed youth with a love of guns who had been undergoing treatment for depression.

Tim Kretschmer, 17, apparently plotted revenge on pupils of his former school in the south-west town of Winnenden because they made fun of him and did not take him seriously.

This was the message he sent to another young man in internet chatroom at 2:45 am (0145 GMT) Wednesday, seven hours before the bloodbath at the Albertville secondary school.

"I mean business, Bernd. I've got a weapon here and I'm going to go to my old school in the morning and fry (them) neatly. Maybe I'll survive ... Take note of the name of the place: Winnenden."

The young man said he did not take the threat seriously and signed off from the chat with the words "laughing out loud." After learning of the shooting, he told his father, who informed police.

Kretschmer was wearing a black combat uniform when he went to the school and shot dead nine students aged 14-15 and three teachers, all but one of them women. He fired a total of 60 shots, picking off his victims in three classrooms.

He then fled, killing three other persons before apparently killing himself 40 kilometres away during a gunfight with police. In all her fired 104 shots in the two incidents.

While on the run he hijacked a car and held the driver hostage, but the motorist was able to jump out of the moving vehicle in a bend and alert police.

Kretschmer, who left school after ending his studies last year, had spent time in hospital undergoing treatment for depression, said Heribert Rech, interior minister the state of Baden-Wuerttemberg where the massacre took place.

He was supposed to continue his treatment at a psychiatric clinic adjacent to the school but broke off the therapy, Rech said.

Kretschmer used a gun he had stolen from the bedroom of the father, a member of a local marksmen's club.

The father owned 15 weapons, including handguns and rifles, all but one of them kept in a safe. The son was apparently well versed in the use of weapons and was often a guest at the club.

Police said the father might have to face charges of involuntary manslaughter because he had failed to lock up the weapon. The parents, who also have a 15-year-old daughter, moved out of the family home on Wednesday to escape media attention.

The Albertville school remained closed as psychologists provided counselling to traumatized students and teachers as well as the families of the victims.

Nine people who were injured in the shooting - five students, two teachers and two policemen - spent the night in the hospital, but their condition was not life-threatening. Three were discharged Thursday morning.

Officials said there had been threats to six other schools in Baden-Wuerttemberg after Wednesday's shooting, prompting police to increase surveillance of school buildings.

The shooting was Germany's second worst school bloodbath since April 2002, when a 19-year-old high school student went on a rampage in Erfurt, killing 12 teachers, two students, a school secretary and a policeman before killing himself.

It also renewed the debate on whether Germany should tighten its gun control laws and install metal detectors at schools.