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.

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.

Swedish police say the shooter who killed 10 people in the country’s worst mass shooting was connected to the adult education center where he opened fire with at least one rifle-like weapon