Stockholm | Sweden's worst mass shooting left at least 11 people dead, including the gunman, at an adult education centre west of Stockholm as officials warned the death toll could rise.

The gunman's motive, as well as the number of wounded, had not been determined by early Wednesday as Sweden — where gun violence at schools is very rare — reeled from an attack with such bloodshed that police early on said it was difficult to count the number of dead among the carnage.

The school, called Campus Risbergska, offers primary and secondary educational classes for adults age 20 and older, Swedish-language classes for immigrants, vocational training and programmes for people with intellectual disabilities. It is on the outskirts of Orebro, which is about 200 kilometres west of Stockholm.

Justice Minister Gunnar Strommer called the shooting “an event that shakes our entire society to its core.” The shooting started Tuesday afternoon after many students had gone home following a national exam. Students sheltered in nearby buildings, and other parts of the school were evacuated following the shooting.

Authorities were working to identify the deceased, and police said the toll could rise. Roberto Eid Forest, head of the local police, told reporters the suspected gunman was among the dead.

There were no warnings beforehand, and police believe the perpetrator acted alone. Police have not said if the man was a student at the school. They haven't released a possible motive, but authorities said there were no suspected connections to terrorism at this point.

Police raided the suspect's home after Tuesday's shooting, but it wasn't immediately clear what they found.

“Today, we have witnessed brutal, deadly violence against completely innocent people,” Prime Minister Ulf Kristersson told reporters in Stockholm late Tuesday. “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.

While gun violence at schools is very rare in Sweden, people were wounded or killed with other weapons such as knives or axes in several incidents in recent years. (AP)