A black cross of mourning appeared on a wall in Winnenden after the massacre
The teenager who attacked a German school and shot dead 15 people said he was doing it "for fun", according to a driver whose car the killer hijacked.
Tim Kretschmer also talked of attacking another school as he forced Igor Wolf, 41, at gunpoint to drive him away, Mr Wolf told Germany's Stern magazine.
Prosecutors are considering whether to charge Kretschmer's father, who owned the gun, with involuntary manslaughter.
The massacre in Winnenden last week plunged Germany into national mourning.
Kretschmer, 17, used a pistol to kill eight schoolgirls, a schoolboy and three teachers at his former school in the small south-western town, and a bystander nearby, before commandeering Mr Wolf's car.
Mr Wolf eventually managed to veer off the road and escape unharmed but the gunman went on to kill two other people in the town of Wendlingen, 40km (25 miles) away before he shot himself as police closed in.
Kretschmer stole the legally held gun from his businessman father's bedroom, police say.
In the interview published on Wednesday, Mr Wolf recounts how the teenager jumped into the back seat of his car after fleeing the school, put a gun to his head and forced him to drive away at high speed.
People grieved publicly outside the Albertville School Centre
He told him he had just killed people at his old school.
"Why are you doing this shit?" the driver asked.
"For fun, because it's fun," Kretschmer replied, speaking very loudly.
When he in turn asked Mr Wolf "Do you think we can find another school?", the driver says he distracted him by changing the subject.
"He was preparing himself for the next shooting - that's what went through my mind," he told Stern.
He initially considered trying to escape from the car at a red light but saw children and women pushing buggies.
"He would have immediately started to shoot, whether at children or old people," Mr Wolf told the German magazine.
A charge of manslaughter is being considered because the father allegedly left the murder weapon unlocked in his bedroom despite knowing his son was depressed.
"There are concrete signs that the parents knew of their son's health problems," prosecutors and local police said in a joint statement.
"Based on this, there is reason to suspect this may be a case of involuntary manslaughter."