The bloody rampage began at around 9:45 a.m. local time (08:45 GMT) when a 17-year-old gunman, later identified as Tim Kretschmer, dressed in black combat fatigues entered the Albertville school in Winnenden, not far from Stuttgart, and opened fire at random.
Nine students and three teachers as well as a person at a nearby clinic were killed in the attack. The gunman, a former student at the school, managed to flee the scene, but was later shot dead by police after an exchange of fire. Two passers-by were also killed and two police officers seriously injured in the shootout.
The exact amount of people injured in the attack has not yet been confirmed.
German Chancellor Angela Merkel has described the shootings as "incomprehensible".
"This is a day of mourning for the whole of Germany," she added.
The gunman finished the Albertville school last year and started vocational studies. Students and teachers have described him as a "completely normal" person.
