This article does not have any sources. (April 2011)
A boy that had been expelled from the school, Robert Steinhäuser, was the person who did all the shooting. In anger, he came into the Gymnasium, and shot dead 12 teachers, a secretary, two students, and a police officer.
After a teacher caught him and put him in an empty room, Steinhäuser shot himself dead. In total, 17 people were killed.