The person was arrested in the morning. Prosecutor Sara Ekström told Expressen that the person is suspected of preparing to commit murder.
The police became aware of the suspected serious crime at 6 p.m. on Monday.
The police quickly identified a suspect and brought them in for questioning. After the questioning, the person was arrested by the prosecutor on duty, police press spokesperson Mikael Hedström said.
One person has been detained, Hedström said, but declined to comment further on the incident.
The high school principal told Söderhamns-Kuriren that the person is a student.
"Yesterday, we learned that one of our students had said something to other students that caused concern and fear of a serious crime," Ann-Charlotte Låks told the newspaper.
The police say there is no longer a threat to the school since the person was arrested. The school remains open and classes are continuing as usual.
The principal has provided information to both students and parents, Mikael Hedström said.