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 have revealed the horrific scene that greeted them when they arrived at the adult education centre in Orebro, Sweden, following a gunman killing eleven people