It was on Sunday morning that an anonymous threat was published on instagram.
The threat was directed at students at the school ISGR in Landala in Gothenburg.
The account where the threat was published must have been known by the school management before, but never published any threats.
On Monday, police had a suspect - a 13-year-old girl.
She was questioned by police in the afternoon, with her parents present.
The police also seized "technical equipment" which they will now examine to secure evidence.
Digitally tracked
The police do not want to go out with how the girl reacts to the suspicions.
She was released after questioning, but is still suspected of making illegal threats against a group.
- We have found her by looking at digital tracks, but I do not know exactly how, says Peter Adlersson, spokesman for the police.
He also says that when it comes to suspicions against such a young person, it is a prosecutor who decides whether to take the person in for questioning or seize evidence.