The German teenager who used his father's pistol to kill 15 people and then himself was a reclusive figure obsessed with ­violent video games, horror films and airguns, police said yesterday as they sought to find a motive for the shootings.

Officials are focusing on Tim ­Kretschmer's state of mind after saying he had struggled with depression and had failed to complete a series of psychiatric sessions last year.

The 17-year-old who shot dead nine pupils, three teachers and three passers-by in an attack on his former school in Winnenden, had become fixated with so-called "shoot 'em up" video games in recent months, spending hours playing one called Counter Strike in which ­players must carry out assassinations to win, police said.

He had also developed a taste for violent films, with a personal collection including Rambo First Blood, while his computer contained some mild pornography.

As flags were flown at half-mast throughout Germany and a one-minute silence was held in schools, investigators tried to sum up the "highly sensitive" teenager who had wreaked terror on Winnenden. "He was quiet, somewhat reserved," said ­Siegfried Mahler, the Stuttgart state prosecutor. "He had some but not many friends. He was interested in girls but had had no serious relationships."

Investigators said one girl Kretschmer had feelings for had not returned them, fuelling speculation that misogynistic resentment may have prompted him to have targeted female victims in his rampage. All except one of the 12 dead at Albertsville secondary school were women or girls. Police would not confirm their identities.

Last night police said they had "doubts" about the veracity of an internet "warning" apparently written by Kretschmer hours before his rampage. The existence of the note, which appeared to have him promising "a real barbecue" at his old school, was denied by the chat forum in which it supposedly featured.

While the motive for the school shooting – Germany's worst since the Erfurt massacre of 2002 – remained unclear, investigators were definite on how the killer had managed to procure a weapon.

The Beretta pistol was taken from his father's collection at home, Mahler said, meaning that Jörg Kretschmer, a member of the local Leutenbach gun club, could potentially face legal action if it was found he had broken laws governing gun storage. He could even be prosecuted for involuntary manslaughter if evidence was found that his son had warned him of his plans.

"Everything here points to negligence on the part of the father as far as the ­storage of this weapon is concerned," said police spokesman Ralf Michelfelder.

On Wednesday morning Kretschmer entered Albertsville brandishing the pistol and more than 250 rounds of ammunition, police said. He is thought to have fired "at least" 60 bullets inside the school, nine in the area of the local psychiatric clinic and a further 44 on the industrial estate where he was eventually caught.

He was found to have 109 unused rounds still on him after he turned the gun on himself.