Austria has declared three days of national mourning following what appears to be the deadliest attack in its post-World War II history.

"I think the first thing is shock, to be honest. I think you can't react the other way. I think everyone in the school is in complete shock," said 24-year-old student Helene Parr, who knows people at the BORG Dreierschützengasse school, where the mass shooting took place.

Residents of Graz gathered for a mass and a candle lit vigil to mourn the 11 victims who were killed after a former pupil opened fire in a school. View on euronews

A 21-year-old man, Arthur A., committed Austria's deadliest school shooting in Graz, killing 10 before turning the gun on himself. Investigations reveal his introverted nature and obsession with online shooting games.

Artur A, 21, carried out the worst school massacre in Austria's history yesterday when he started shooting students at a school in Graz, where he had reportedly been the target of bullying

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.

A teacher and students were killed during the attack in second largest city Graz on Tuesday by gunman with two firearms

By Francois Murphy and Max Schwarz GRAZ, Austria (Reuters) -Austrian police on Thursday described the 21-year-old man who carried out Austria's worst school shooting as an introvert who had largely withdrawn from the outside world before he carefully planned the attack.