Three years to the day a disgruntled youth shot dead 16 people at his former high school before turning the gun on himself, the institution reopened its doors for the first time Tuesday for an emotional memorial ceremony. Church bells in the eastern German city of Erfurt rang out at exactly 10:58 am (0858 GMT), the precise time at which 19-year-old Robert Steinhäuser began a shooting rampage through the Gutenberg high school, killing 12 teachers, a secretary, two students and a police officer. Sobbing former students and teachers left roses at the school's entrance before entering the renovated scene of the bloodbath. "For many of us, it is a return that we have to work through," former principal Christiane Alt said. Germans were shocked by the incident, in which the black-clad, mask-wearing Steinhäuser, armed with a pistol and a pump-action shotgun, went on a killing spree, apparently in revenge for having been expelled. At the Erfurt cathedral, local residents left candles, personal notes and other mementos beneath photos of the victims. Many of the pupils who attended the school at the time of the assault are still undergoing psychological treatment. They will remain in the provisional premises used for the last three years until July, shortly before the summer recess, when the striking Art Nouveau school building will officially reopen.