The attacker struck at a secondary school in the central Swedish town of Fagersta, also severely injuring two teenage boys before being arrested by police.
The parents of a 17-year-old girl told local newspaper Fagersta-Posten that their daughter was killed in the attack.
Police are investigating whether a TikTok account that posted a picture of a sword 20 minutes before the assault belonged to the suspect, the source told Reuters.
According to the Dagens Nyheter newspaper, the image appeared to have been taken in a bathroom at the school.
Before it was taken down on Friday, the month-old account contained videos that referred to two episodes of mass violence in Sweden and to Norwegian mass murderer Anders Behring Breivik.
One of the videos referenced a 2015 attack in which a masked swordsman killed a teaching assistant and two boys at a school in Trollhattan, western Sweden, before being shot dead by police.
"The investigators are looking into various online communities which might have egged on the suspect to carry out the attack," the police source said.
Swedish Radio reported that the suspect was a former pupil of the school and had an access card to the site.
Several party leaders cancelled planned speeches on Saturday, pausing campaigning ahead of the September 13 election.
Prime Minister Ulf Kristersson and opposition leader Magdalena Andersson travelled to Fagersta to offer condolences to the bereaved family and visit the injured.
"What should never happen has happened again," Kristersson told reporters in Fagersta.
"Our thoughts go first and foremost to the families who have been incredibly hard hit by this," he said.
Sweden's worst mass shooting occurred in February 2025 at a school in the south-central city of Orebro, when a gunman killed 10 people before turning the gun on himself.
On Saturday, the police clarified that the suspect in the Fagersta attack had not been shot, contrary to what they had said on Friday.
"Service weapons were used but the perpetrator has not been injured in connection with the arrest, and has not been in need of medical attention," a police spokesperson told the Aftonbladet daily.
Prosecutors ordered the man's detention on suspicion of murder and several counts of attempted murder.
"The investigation is at an intense stage. We are conducting interrogations and gathering information but I cannot go into specifics," prosecutor Ann-Sofie Trossing told Reuters.
with DPA
