The father of a teenager who killed 15 people last week is being investigated on involuntary manslaughter charges for failing to secure the pistol used in the attack, German authorities said Monday.

Authorities have said 17-year-old Tim Kretschmer used a 9mm Beretta pistol that belonged to his father. The man kept the gun unsecured in his bedroom in violation of German law, they said. The father's other 14 firearms were properly secured.

The man's full name has not been released.

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A joint statement by police and prosecutors said the father apparently knew that his son had received psychiatric treatment for depression.

"There are concrete signs that the parents had knowledge of the health problems of their son," the statement said. Kretschmer's parents, through their lawyer, have played down the seriousness of their son's ailment.

Kretschmer killed nine students, three teachers and three others before taking his own life last week.

Prosecutors have said that Kretschmer was treated five times last year for depression in a psychiatric clinic. He was advised to continue the treatment but failed to do so.