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.

Sweden is reeling from the country's deadliest mass shooting this week as investigators continue to look into the gunman's motive.