Police and state prosecutors announced Saturday that the 21-year-old man from the town of Osterholz-Scharmbeck in Lower Saxony had stabbed the teacher, 35, more than 20 times. She died at the scene of the attack – a carpark close to her home.

Police said the man had wanted to ask “several thousand questions” of his former teacher and declared his love for her before he attacked.

A witness tried unsuccessfully to stop the attack.

The woman taught biology and chemistry at a high school in Osterholz-Scharmbeck. She had previously contacted police in January 2008 because she feared he might try to kill her.

Police investigated the man – who was considered a peculiar loner – at the time and found in his house gunpowder and fireworks. But they found nothing with which they could prosecute him.

The former pupil called the police himself after the murder and then waited to be arrested.

State prosecutor Uwe Picard said he gave his name and spoke lucidly during the call, telling police: “I’ve killed someone. I’ve put down my weapon. Please pick me up.” Police were still questioning the man on Saturday.

He had apparently planned the attack well in advance. During his interrogation, he said he had prepared several questions on various topics – including intimacy and sex – which he wanted to put to his former teacher.

The young man told police he had fixed a GPS tracking device to the teacher’s car about 10am on Friday and used it to follow her to the car park near her home where he confronted her.

He had planned to force her into a neighbouring house but this fell through when the woman resisted.

The man, who wasn’t awarded his high school certificate, had apparently been spying on the teacher for some time without her knowledge.