At least 10 people were reported dead in a shooting at a school in the central Swedish city of Orebro on Tuesday.
It was the "worst mass school shooting in Swedish history," Prime Minister Ulf Kristersson said at a news conference.
Members of the public were urged to stay away from the area, or stay inside their homes as the investigation unfolded.
The gunman was later found dead.
"Sweden has "never seen a school shooting of this magnitude," Justice Minister Gunnar Strommer said, according to the BBC.
School attacks are relatively rare in Sweden, which has in recent years grown more accustomed to shootings and bombings linked to gang violence that kill dozens of people each year.
Students in nearby schools and the school in question had been locked in "for safety reasons," police said.
According to several Swedish media, witnesses reported hearing what they believed to be automatic gunfire.
Newspaper Aftonbladet wrote that it had received reports that the local hospital had emptied its emergency room and intensive care unit in anticipation of the wounded.
No motive was known for the shooting.
Though school attacks are rare, several serious incidents have taken place at schools in recent years.
In March 2022, an 18-year-old student stabbed two teachers to death at a high school in the southern city of Malmo.
Two months earlier, a 16-year-old was arrested after wounding another student and a teacher with a knife at a school in the small town of Kristianstad.
In October 2015, three people were killed in a racially-motivated attack at a school in the western town of Trollhattan by a sword-wielding assailant later killed by police.
Schools were due to reopen on Wednesday.
AFP contributed to this report.
