A gunman who killed 11 people, including himself, at an adult education centre in central Sweden may have been a student at the school, police said on Thursday, as they described chaotic scenes after the country's deadliest mass shooting as being like an "inferno".

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.

Police investigating Sweden's worst mass shooting were still trying to establish a motive as to why a lone gunman opened fire at an education campus in the southern city of Orebro, killing 10 people.