The girl has been in the same online environment as the suspected Fagersta murderer, the Prosecution Authority writes in a press release. She is not suspected of involvement in the school attack.
The police have carried out a search at her home and seized items that are now being analysed. The girl has subsequently been taken into the care of social services, says prosecutor Adolf Oleszkiewicz in the press release.
The girl denies the crime, according to Sveriges Radio.
The police have also taken into custody a 13-year-old boy who had contact with the suspected Fagersta murderer. The boy is suspected of making a serious unlawful threat.
A 16-year-old has previously been detained for aiding and abetting murder and attempted murder.
