Police in Austria were hunting for clues on Wednesday after a gunman opened fire at his former high school a day earlier, killing nine students and a teacher in a deadly spree that stunned the country.

Austrian authorities were searching on Wednesday for clues to why a 21-year-old gunman shot dead 10 people in a rampage at his former high school before killing himself, in one of the worst outbreaks of violence in the country's modern history.

The Austrian man, identified by local media as Arthur A., killed 10 people before shooting himself at his former high school in the southern city of Graz, sending shockwaves through Austria, which declared three days of national mourning.