Several media reported the suspected gunman turned his gun on himself -- but police have not confirmed those reports.
"Eleven people are dead, including the killer," police told AFP about Tuesday's massacre at Campus Risbergska, a secondary school for young adults.
Local health authorities said Wednesday six people were being treated at Orebro's university hospital.
Five of them -- three women and two men -- had undergone surgery for gunshot wounds and were in a "stable but serious" condition.
A woman was also being treated for minor injuries, Orebro County authorities said in a statement, adding that all the injured were over the age of 18.
"This is the worst mass shooting in Swedish history," Prime Minister Ulf Kristersson told a press conference late on Tuesday.
Kristersson noted a lot of "questions were still unanswered".
"But there will come a time when we will know what happened, how it could happen and what motives may have been behind it," Kristersson said, urging people not to "speculate".
"The motive for the shooting is not yet known, but all the indications are that the perpetrator acted alone, without ideological motive," Orebro police said in a statement.
Police have not disclosed any information about the identity or ages of the dead, nor whether they were students or teachers at the school.
School attacks are relatively rare in Sweden, but the country has suffered shootings and bombings linked to gang violence that kill dozens of people each year.
"The perpetrator is not known to the police, he has no gang affiliations. We believe there will be no further attacks," Orebro police chief Roberto Eid Forest told reporters on Tuesday evening.
- 'Numb, speechless' -
Swedish television channel TV4 reported that police had raided the suspect's home in Orebro late on Tuesday afternoon.
It said the suspect was around 35 years old and had a license to carry a weapon and no criminal record, but did not provide any details about his identity.
The man lived reclusively, was unemployed, and had distanced himself from his family and friends, newspaper Aftonbladet reported, citing family members.
The shooting struck around midday on Tuesday.
"I was standing there, watching what was happening, and I was just around here when I saw some bodies lying on the ground. I don't know if they were dead or injured," 16-year-old Linn, who goes to school near the site of the massacre, told an AFP correspondent at the scene.
"There was blood everywhere, people were panicking and crying, parents were worried... it was chaos," she added, her voice trembling.
Liv Demir, 36, whose son attends a school near the Campus Risbergska, said she was shocked to hear of the shooting.
"I became numb, speechless. I didn't really know where to go," she told AFP on Wednesday morning.
Her son also has gym classes at Campus Risbergska.
"So my thoughts were spiralling because I packed his sports bag in the morning," Demir said.
- 'Dark hour' -
Sweden's King Carl XVI Gustaf said in a statement Tuesday that he had received the news of the shooting with "sadness and dismay."
The Royal Court announced Wednesday that flags would be "flown at half-mast at all royal palaces" during the day. The government announced a similar measure for its own offices and parliament.
European Commission president Ursula von der Leyen described the shooting as "truly horrifying".
"Such violence and terror have no place in our societies -- least of all in schools. In this dark hour, we stand with the people of Sweden," she said in a post on X.
Though such shootings are rare, several other violent incidents have struck Swedish schools in recent years.
In March 2022, an 18-year-old student stabbed two teachers to death at a secondary 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 who was later killed by police.
© Agence France-Presse
