Suspect device found in deadly Stockholm attack truck

Stockholm: Swedish police found a suspect device in the truck that ploughed into a Stockholm crowd killing four in Europe's latest such terror attack, officials said on Saturday.

The alleged driver, who is in custody, is a 39-year-old man from Uzbekistan who was already known to Swedish police, authorities said.

"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.

Intelligence agency chief Anders Thornberg added that the Uzbek suspect "has appeared in our intelligence gathering in the past".

"There is nothing to indicate that we've got the wrong man. On the contrary, the suspicions have strengthened," Eliasson added.

Flags flew at half-mast across Stockholm today as the city slowly returned to normal a day after the attack.

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 today.

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).

Prime Minister Stefan Lofven, who said Sweden will hold a minute's silence on Monday in memory of the victims, has beefed up Sweden's border controls.

"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 today, 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."

The suspected driver was detained on Friday in Marsta, a suburb north of Stockholm. According to several media outlets, he is an IS supporter.

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.

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.