The father of a teenager who shot dead 15 people then himself in a rampage at his old school went on trial Thursday, awakening painful memories of a bloodbath that shocked Germany.

Joerg Kretschmer, a 51-year-old businessman, stands accused of violating gun laws, enabling his son Tim, 17, to take his 9mm Beretta pistol in March 2009 and use it to deadly effect in the southern town of Winnenden.

Fearing a break-in, Joerg Kretschmer kept his gun by the bedside, rather than locking it away, a mistake prosecutors say cost the lives of nine of Tim's fellow students, three teachers and three others during a dramatic chase.

The court in nearby Stuttgart must decide to what extent the father was responsible for his son's crimes. There is strong evidence to suggest that Tim could have broken into his father's gun chest had he wished, authorities say.

Prosecutors initially pressed charges of manslaughter and grievous bodily harm but a court ruled that these more serious charges could only be considered if his actions had directly caused the massacre.

With 40 witnesses over 27 days, the trial promises to be one of the biggest in the court's history. Some 41 co-plaintiffs, represented by 19 lawyers, are also expected to participate in the case.

In the wake of the disaster, the German government introduced an amnesty on illegally held weapons and more than 200,000 guns were handed in. But as the trial opened, a leader of the Green party said not enough had been done.

"Even after Winnenden and despite demands from all parties, the necessary actions were not taken," Cem Ozdemir, co-chairman of the Greens, told the Frankfurter Rundschau daily.

If convicted, Joerg Kretschmer faces a fine or suspended sentence.

Relatives of those killed said the trial is not about settling scores but about discovering how much Tim's parents knew about their son's psychological problems.

"It is not about revenge," said Hardy Schober, who lost his 15-year-old daughter in the bloodbath.

"The truth must come to light, no matter how painful it is," he told the Frankfurter Rundschau.