The person was arrested late Monday evening, according to the Swedish Prosecution Authority.
"The investigation is at an early stage, and the police have seized storage media that are now being analyzed. The police are securing evidence and will interview the suspect and witnesses," prosecutor and preliminary investigation leader Sara Ekström said in a press release.
The police became aware of the suspected serious crime at 6 p.m. on Monday.
The police quickly found a suspect and brought the person in for questioning. After the questioning, the prosecutor on duty arrested the person, police press spokesman Mikael Hedström said.
One person has been detained, Hedström said, declining to give further details about the incident.
The high school principal told Söderhamns-Kuriren that the person is a student.
"Yesterday, we learned that one of our students had expressed themselves to other students in a way that raised concerns about a serious crime," the principal told the newspaper.
The police assess that there is no threat to the school now that the person has been arrested. The school remains open, and classes are continuing as usual.
The prosecutor must decide by noon on Thursday whether the person should be remanded in custody or released.
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