Updated

BERLIN (AP) — The father of a 17-year-old who went on a rampage at a German school killing 15 people and then himself is on trial on charges he did not properly secure the weapon used in the attack.

Joerg Kretschmer is accused of leaving the 9mm pistol used by his son Tim in the 2009 attack in the town of Winnenden unsecured in his bedroom.

His trial opened Thursday in Stuttgart state court and is expected to last 27 days. If convicted he could face a maximum three years in prison or a fine.

Tim Kretschmer killed nine students and three teachers at his former high school before killing three others elsewhere. He then killed himself during after exchanging shots with police.

The court says 41 relatives of victims of the attack have joined the trial as co-plaintiffs as allowed under German law.