Shocked and grieving residents in the German city of Erfurt are struggling to understand why a failed student shot dead 16 people before killing himself in the country's worst murder spree in post-war history. The 19-year-old returned to his high school on Friday armed with a pump-action shotgun and handgun to kill two pupils, 13 teachers, a police officer and then himself. Another six people were wounded. A German police source on Saturday identified the killer as Robert Steinhaeuser -- not Robert Steinhof as some media had initially named him. A police spokeswoman also said 13 teachers were killed in the attack, not 14 as initially reported, bringing the death toll down to 17 from 18.