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."

Police believe the perpetrator acted alone but haven't said by early Wednesday if he was a student or what his motive may have been.

OREBRO, Sweden (AP) — Sweden’s worst mass shooting left at least 11 people dead, including the gunman, and at least five seriously wounded at an adult education center west of Stockholm. The gunman’s motive hadn’t been determined by early Wednesday as the Scandinavian nation — where gun violence at schools is very rare — reeled from an attack.

Sweden’s worst mass shooting has left at least 11 people dead, including the gunman, at an adult education center as officials warned that the death toll could rise

Students and teachers recalled trying to save the lives of their comrades before fleeing the scene of a mass shooting in Sweden described by the prime minister on Wednesday as a "dark day" in the country's history.

A gunman opened fire at an adult education center in the Swedish city of Örebro on Tuesday, killing at least 10 people in what the country’s prime minister called the “worst mass shooting in Swedish history.

Sweden's King Carl XVI Gustaf and Queen Silvia on Wednesday visited the grounds of the education centre where 11 people were killed on Tuesday in the Nordic country's worst mass shooting. (Reporting by Johan Ahlander and Anna Ringstrom,