It's pretty tragic, and yes, it has indeed happened. I don't have any sympathy towards the people who committed this crime. While it's likely that they're muslim since they're from North Africa, Swedish police don't register what religion people belong to when arresting people. They're street-kids after all, so not sure how much religion they've been able to take in. So the fact that they're "absolutely muslim" is nothing that has even been stated by the police, it's just something the foreign papers (?) have added (I read the article in several credible Swedish newspapers, their religion was never mentioned). Sidenote: I despise all abrahamic religions, I wish none of them would have any political, social or cultural influence on any part of the world.
There's a right-wing surge in the nordic countries at the moment that follows the global western world momentum. Not surprisingly, the sites that you follow probably did not report about the white Swedish teenager who brought swords to an immigrant-majority school and started slicing people up, resulting in the deaths of three people, one of them being a student and the others teachers trying to protect the students.
In addition to this, your site also probably didn't report that this year alone, there have been 17 arsons at locations that were set up for refugees. The locations of these houses were posted by the Swedish Democrates, an anti-immigration party in Sweden, and it became pretty obvious that they wanted people to keep burning down these buildings since they just kept posting the addresses even after the first arsons. One member of this party was also found, along with a well-known nazi, with 550 kg dynamite in their possession. While I agree that some refugees are dangerous, they are few, and if stastics show us anything, the right-wing side of Sweden is probably something we should be a lot more scared of.