Officials in Sweden have vowed to determine why a man killed 10 people, and apparently himself, in an unprecedented attack that's shaken their "society to its core."

Everything we know so far about campus attack which left 11 dead - Sweden’s prime minister said the attack at the Risbergska School was the worst mass shooting in country’s history

The police responding to the attack encountered firing so intense that they struggled to tell how many gunmen were on the scene, an official said.