Friday's attack was the latest in a string of similar assaults with vehicles in Europe, including in London, Berlin and the southern French city of Nice. Photograph: (AFP)
The man was arrested 'on suspicion of a terrorist crime through murder', a Swedish Prosecution Authority official said
Swedish police have found a suspect device in the truck that slammed into a crowd in central Stockholm, killing four people and injuring 15, authorities said on Saturday.
"We have found a device in the vehicle that doesn't belong there ... A technical examination is ongoing, we can't go into what it is right now ... whether it's a bomb or a flammable device," police chief Dan Eliasson told reporters.
Flags flew at half-mast across Stockholm on Saturday as the city slowly returned to normal a day after the truck attack, as police said they had the suspected driver in custody.
A stolen beer truck ploughed into a crowd of people at the corner of the bustling Ahlens department store and the Drottninggatan pedestrian street on Friday afternoon, above ground from Stockholm's central subway station.
Fifteen people were injured, nine of whom remained in hospital on Saturday.
It was the third terror attack in Europe in two weeks, coming on the heels of assaults in London and St. Petersburg, although there has been no immediate claim of responsibility.
Previous attacks using vehicles have occurred in London, Berlin and the southern French city of Nice, all of them claimed by the so-called Islamic State (IS).
"Terror hits the heart of the city," Sweden's biggest broadsheet Dagens Nyheter headlined its front page above a picture of the truck with its front end smashed into the store.
Prime Minister Stefan Lofven said he had strengthened the country's border controls, as flags flew at half-mast at parliament, the royal palace, the government offices, and City Hall.
"Terrorists want us to be afraid, want us to change our behaviour, want us to not live our lives normally, but that is what we're going to do. So terrorists can never defeat Sweden, never," Lofven said.
City streets were empty early Saturday, slowly filling as the day wore on as things began to return to normal -- apart from a heavy police presence, a rare scene in this normally tranquil country.
A swelling crowd milled by the security barrier erected around the scene, many placing flowers on the ground or in the security fence.
Sweden's Crown Princess Victoria, 39, was one of those laying down a bouquet, wiping tears from her cheek.
"I feel an incredible sadness, an emptiness," she told reporters.
But, she said, "society has demonstrated enormous strength and we stand together against this."
Suspected driver in custody
Swedish police said a man arrested on "suspicion of a terrorist crime" was probably the truck driver.
"We suspect that the man who was arrested is the perpetrator," Stockholm police spokesman Lars Bystrom told AFP.
The man was arrested "on suspicion of a terrorist crime through murder," Karin Rosander, spokeswoman at the Swedish Prosecution Authority, said.
The man was detained on Friday in Marsta, a suburb north of Stockholm. According to several media outlets, he is a 39-year-old of Uzbek origin and an IS supporter.
Prosecutors did not disclose his identity, but police said his appearance "matched the description" of a photo they released of a suspect wearing a dark hoodie and military green jacket.
Intelligence agency Sapo said meanwhile it was hunting for "possible accomplices or networks that may have been involved in the attack."
Witnesses described scenes of terror and panic on Friday.
"A massive truck starts driving ... and mangles everything and just drives over exactly everything," eyewitness Rikard Gauffin told AFP.
"It was so terrible and there were bodies lying everywhere... it was really terrifying," he added.
Passerby Hasan Sidi told Aftonbladet he saw two elderly women lying on the ground.
He said people at the scene urged him to help one of the women who was "bleeding to death".
"One of them died... I don't know if the other one made it," Sidi said.
The truck was towed away in the early hours of Saturday.
'It was expected'
An attack on Stockholm was just a matter of time, the head of the Centre for Asymmetric Threat Studies at the Swedish National Defence College, Magnus Ranstorp, told AFP.
"It was pretty expected, the police and intelligence agency have practised for this several times the past year... We just didn't know when it was going to happen," he said.
Friday's attack was the latest in a string of assaults with vehicles in Europe.
The deadliest came last year in France on the July 14 Bastille Day national holiday, when a man rammed a truck into a crowd in the Mediterranean resort of Nice, killing 86 people.
In December, a man hijacked a truck and slammed into shoppers at a Christmas market in Berlin, killing 12 people.
In London last month, Khalid Masood, a 52-year-old convert to Islam, killed five people when he drove a car at high speed into pedestrians before launching a frenzied knife attack on a policeman guarding parliament.
In 2014, IS called for attacks on citizens of Western countries and gave instructions on how they could be carried out without military equipment, using rocks or knives, or by running people over in vehicles.
Friday's attack was the second terror attack in Stockholm.
In December 2010, a suicide bomber blew himself up, also on the Drottninggatan pedestrian street, lightly injuring several passersby.