Winnenden, Germany  - The teenager who killed 15 people in a school shooting in Winnenden, south-west Germany might not have revealed his intentions in advance, police said on Thursday.

Earlier, officials had said the youth, identified as Tim Kretschmer, 17, had told another person in an internet chat forum that he planned to kill pupils at his former school because they had made fun of him.

Police said reports that information to this effect was found on his computer turned out to be false. It was not clear why. A police spokesman said there was no such information on the computer.

The interior minister of Baden-Wuerttemberg, the state where the school is located, told a press conference earlier Thursday that police had evidence Kretschmer revealed his intentions hours before Wednesday's shootings.

Interior Minister Heribert Rech even quoted statements purportedly made by the gunman that he had a weapon and intended to go to the school and "fry" some of the pupils.

Police said they were questioning witnesses who reported seeing the entry on the chat forum to clarify if and when such a threat was actually made.

They were also in touch with the US operators of the server on which the chat forum is based to see if they can determine when the entry was made.

Kretschmer killed nine students and three teachers at the Albertville secondary school where he had been a pupil until last year. He then fled, killing three other people before shooting himself when caught by police after a 40-kilometre chase.

Officials said Thursday the youth had been under psychiatric treatment for depression. dpa