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 said they were still investigating the motive behind the country's deadliest mass shooting, as local media began reporting details about the gunman. The suspect, named in reports as 35-year-old local man Rickard Andersson,