Berlin - Rules governing the private ownership of guns need to tightened, a senior German official said Friday, two days after a teenager killed 15 people in a shooting spree.

"The way owners store their weapons is a critical matter," Deputy Interior Minister August Hanning said at the opening of an exhibition of hunting and sporting guns in the Bavarian city of Nuremberg.

He said it was necessary to make greater use of locking mechanisms developed by the firearms industry to restrict access to weapons. He also called for a debate on whether biometric data should be introduced in security systems for weapons and gun cabinets.

The Beretta pistol and ammunition used by 17-year-old Tim Kretschmer to gun down pupils at his former school in the town of Winnenden on Wednesday were stolen from his father's bedroom.

The father, an avid marksmen, owned 15 weapons, including handguns and rifles, all of them but the Beretta locked up in a safe. He also had 4,600 rounds of ammunition in his home.

There are about 2.5 million weapons kept in private ownership in Germany.

The killings sent shock waves through the country, coming seven years after another school shooting in the eastern town of Leipzig claimed 17 lives.

Police Friday were still trying to fathom what prompted Kretschmer to go on the rampage, killing nine schoolchildren, three teachers and three other people before turning his gun on himself during a shootout with police.

The teenager, described as a troubled young man with a love of guns and violent computer games, had been undergoing treatment for depression, but recently broke off the therapy.

Claims that he posted a warning on an internet website six hours before the shootings could not be confirmed by police on Friday. The website denied the existence of such a message.

Kretschmer was wearing a black combat uniform when he went to the Albertwille secondary school and shot dead his victims, all but one of them women. Kretschmer had left the school last year after completing his studies.

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

Relatives of the dead pupils and teachers were able to have a final glimpse of their loved ones as they were laid out in open coffins at a town hospital on Thursday evening. (dpa)