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Police on Saturday said they had arrested a man from Uzbekistan suspected of plowing a truck into a department store in the Swedish capital, killing four people and injuring 15.

Authorities confirmed that the 39-year-old suspect was known to security services and described him as a "marginal character." They would not confirm his status in Sweden.

Speaking at a press conference, the head of Sweden's National police, Dan Eliason, said security officials had found an object in the truck that "could be a bomb or an incendiary object."

The suspect was detained Friday and matched an image put out by police showing a man in a green jacket, hoodie and white shoes, according to a televised police statement.

Eliason also said that police could not rule out that several people were involved in the attack.

The incident sent shock waves through the country. Sweden's crown princess Victoria held back tears as she left flowers for the victims at the site of the attack on Saturday.

"Sweden shall come out stronger from this, and we get through this together," she told the press and mourners gathered near the department store.

"It hit people — it was terrible. It hit a pram with a kid in it, demolished it."

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A statement on the royal website said the king and queen of Sweden had interrupted their trip to Brazil and were returning home.

Later Saturday, King Carl XVI Gustaf thanked the emergency services and said his thoughts were with the victims.

Addressing the Swedish people in a statement, he said: "We are all shocked by what has happened. At the same time, it inspires hope to see the consideration people have shown each other and it manifests the strength in our society."

He added: "Unfortunately, Sweden has been a victim of acts of violence before, but we overcame it then, and we are going to overcome it now. Sweden is, has long been, and shall continue to be a safe and peaceful land."

Swedish Prime Minister Stefan Löfven said Saturday that his country was in sorrow and said Monday would be a day of mourning in Sweden. But he said Sweden would continue to be an open society.

"Sweden has shown its best side during this attack," Löfven said on state television at the scene of the attack. "To help each other, support each other, and open our homes to other humans. We want to have an open society in Sweden. We shall continue with that."

Sweden's Vice Prime Minister Isabella Lovin and Education Minister Gustav Fridolin also laid flowers close to the site of the attack, The Associated Press reported.

Earlier, prosecutor Hans Ihrman said a person had been formally identified as a suspect "of terrorist offenses by murder," and should face a pretrial custody hearing before midday Tuesday or be released, the AP reported.

Stockholm's Karolinska hospital told the news agency six of those injured in the attack had been released and eight adults and one child were still in the hospital.

A truck that crashed into the Ahlens department store in central Stockholm is towed from the site early Saturday. MAJA SUSLIN / AFP - Getty Images

Police launched a manhunt in Stockholm after a truck plowed into a crowd on Stockholm's busy Drottninggatan shopping district before barreling into the side of the Ahlens store, the country's security agency said. Panicked pedestrians and shoppers scrambled for safety.

On Friday, Löfven said that "everything indicates that this is a terrorist act."

Glen Foran, an Australian tourist in his 40s who witnessed the attack, told Reuters that he "turned around and saw a big truck coming towards me. It swerved from side to side. It didn't look out of control. It was trying to hit people."

"It hit people — it was terrible. It hit a pram with a kid in it, demolished it," he said on Friday.

"It took a long time for police to get here. I suppose from their view it was quick, but it felt like forever."

The area of the attack in central Stockholm was evacuated, including the main rail station, and remained cordoned off late on Friday. All subway traffic was halted on police orders and government offices were closed.

Police recommended all travelers carry identification as officials tightened Sweden's border controls. Authorities also advised people to avoid central Stockholm due to irregular traffic while the area around the attack site remained cordoned off.