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.

Witnesses who survived Sweden’s worst mass shooting have described horror and panic that gripped an adult education center west of Stockholm as the gunman killed at least 10 people.

Swedish police are still investigating the gunman's personal history and have not said the attack, which killed 11 people, was targeted. Hellen Werme, a 35-year-old trainee nurse, survived the shooting. She and her classmates locked themselves inside their classroom as the shooter prowled the corridors.