From Sky News:
"Shortly after the shooting, a man who claimed to be the gunman released a video on social media in which he claimed to be a member of Islamic State (IS) and a "fighter for Allah".
He also claimed he had carried out the attack in "revenge in the name of Muslims"."
If this is the case, it is probably retaliation from a nutjob over the provocative Koran burnings that has been going on in Sweden.