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 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".

Police say around 10 people have died in a shooting at an education campus in the city of Orebro. It is the worst school shooting ever to happen in Sweden, where school violence is rare. The suspected gunman is believed to be among the dead.

The authorities said that at least 11 people were killed and that the suspect was believed to be among the dead.

Sweden on Wednesday mourned the victims of its worst mass killing as police indicated that the gunman likely shot himself after killing 10 people at an adult education centre.