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.

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 suspect, named in reports as 35-year-old local man Rickard Andersson, was reported to be a former student of the school in Orebo, a city 157 km (98 miles) west of Stockholm, where the attack took place on Tuesday. Eleven people died in the shooting, including the attacker, with at least six others injured.