Around ten people, including the gunman, were killed on Tuesday at an adult education centre in what Sweden’s prime minister called the country’s “worst mass shooting”.
A final death toll and a conclusive number of wounded has not yet been determined.
Prime Minister Ulf Kristersson gave a news conference hours after the tragedy.
“Today, we have witnessed brutal, deadly violence against completely innocent people,” Mr Kristersson told reporters.
“This is the worst mass shooting in Swedish history. Many questions remain unanswered, and I cannot provide those answers either.
“But the time will come when we will know what happened, how it could occur, and what motives may have been behind it. Let us not speculate,” he said.
The damage at the crime scene was so extensive that investigators were unable to be more definitive, said Roberto Eid Forest, head of the local police.
The shooting happened on the outskirts of the city of Orebro, which is about 125 miles west of Stockholm.
Police said that the death toll could rise.
Mr Eid Forest told reporters that the suspected gunman was among those killed. Police believe the perpetrator acted alone, and he was not previously known to police, officials said.
Authorities said that there were no suspected connections to terrorism at this point, but police did not provide a motive.
The school, called Campus Risbergska, serves students who are over 20, according to its website.
Primary and upper secondary school courses are offered, as well as Swedish classes for immigrants, vocational training and programmes for people with intellectual disabilities.
Gun violence at schools is very rare in Sweden. But there have been several incidents in recent years in which people were wounded or killed with other weapons such as knives or axes.
Justice minister Gunnar Strommer called the shooting “an event that shakes our entire society to its core”.
While Swedes read about such violence in other places, Mr Strommer said that the country previously felt it would not happen there.
Other tragedies in Swedish schools were not to the extent of Tuesday’s attack, he said, calling it “indescribably sad” for the community.
The shooting also sent shockwaves through Europe and officials in Brussels expressed their outrage at the carnage.
“What happened today in Orebro is truly horrifying,” European Commission President Ursula von der Leyen wrote on social media.
“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.”
Police raided the suspect’s home after Tuesday’s shooting, but it was not immediately clear what they found.
Mr Eid Forest said there were no warning signs before the attack and authorities were working to identify the deceased.
Earlier, Mr Kristersson said that it was “a very painful day for all of Sweden”, Swedish media reported.
“My thoughts are also with all those whose normal school day was exchanged for terror,” Mr Kristersson said.
“Being confined to a classroom with fear for your own life is a nightmare that no one should have to experience.”
Swedish King Carl XVI Gustaf praised police and the rescue and medical personnel who responded to the shooting, and offered words of comfort to the families of the victims.
“It is with sadness and dismay that my family and I have received the information about the terrible atrocity in Orebro,” the king said in a statement.
“We send our condolences tonight to the families and friends of the deceased. Our thoughts at this time also go to the injured and their relatives, as well as to others affected.”
The shooting erupted after many students had gone home after a national exam.
Police vehicles and ambulances, lights flashing, blanketed the car parks and streets around the school as a helicopter buzzed overhead.
Teacher Lena Warenmark told SVT News that there were unusually few students on the campus after the exam. She also told the broadcaster that she heard probably 10 gunshots.
Students sheltered in nearby buildings. Other parts of the school were evacuated following the shooting, which began at about 12.30pm local time.
Andreas Sundling, 28, was among those forced to barricade themselves inside the school.
“We heard three bangs and loud screams,” he told Expressen newspaper while sheltering in a classroom.
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