Ten people, including a child, are still being treated in hospital after the Stockholm truck attack, as police search properties and continue to question a 39-year-old native of Uzbekistan over the incident.

Key points: Suspect previously known to intelligence services as marginal figure with no clear links to extremist groups

Suspect previously known to intelligence services as marginal figure with no clear links to extremist groups 10 people remain in hospital, including two adults in intensive care

10 people remain in hospital, including two adults in intensive care Monday will be day of mourning for victims, Sweden's PM announces

Four people were killed on Friday (local time) when a stolen delivery truck crashed into crowds in central Stockholm.

Of the 10 people still in hospital, two adults remain in intensive care.

Police have raided several addresses in the Stockholm area but said no further arrests in connection with the attack had been made.

Officers were seen leaving an apartment in Varberg, south-west of Stockholm, with items including what appeared to be a laptop bag.

The Uzbek man being held in custody is the suspected driver of the hijacked delivery truck.

He was previously known to Swedish intelligence services as a marginal figure with no clear links to extremist groups.

A police photographer could be seen during a house search as part of the Swedish investigation. ( Reuters/TT News Agency )

"Nothing indicates that we have the wrong person, on the contrary, suspicions have strengthened as the investigation has progressed," said Dan Eliasson, the head of Sweden's national police.

"We do not know whether there are further persons involved in this act or not, but we are not excluding that. We are still working on a very comprehensive approach."

Authorities have not yet pointed to a motive for the attack.

Uzbekistan and other former Soviet Central Asian republics have long been a recruiting ground for Islamic militant groups, notably the Islamic Movement of Uzbekistan, which formed in 1998.

Originally allied with Al Qaeda, many of the group's fighters have switched to Islamic State group affiliation.

Police said they had found a suspicious device in the truck, which ended up rammed into the Ahlens department store.

However, they did not yet know if it was a homemade bomb, as reported by public broadcaster SVT.

"We have found something in the truck in the driver's seat, a technical device which should not be there. I cannot at this stage say that this is a bomb or some sort of flammable material," Mr Eliasson said.

A makeshift memorial takes shape on the fencing erected outside the Ahlens department store. ( Reuters/TT News Agency )

As the police investigation continues, thousands of people have been visiting the site of the attack to pay their respects.

There are flags, flowers, cards and candles around large parts of the crime scene in central Stockholm.

Behind the wall of flowers, a gaping hole in the wall of the department store showed the force of the impact from the truck, which was removed for examination by forensics experts.

Visiting the scene, Sweden's Crown Princess Victoria laid roses on the ground and wiped away a tear.

"We must show a huge force, we must go against this," she told reporters. "Swedish society is built on huge confidence, a sense of community."

On Monday (local time), Sweden will hold a minute's silence to mourn the dead.

Swedish society is built on huge confidence, says Crown Princess Victoria. ( Reuters/NTB Scanpix )

Swedes question immigration policies in wake of attack

The attack has also raised the question of whether Swedish society, considered democratic and egalitarian, has failed to integrate its newcomers.

But it was unclear whether the suspect was also a Swedish citizen or resident, or even how long he had been in the country.

"We have been too liberal to take in people who perhaps we thought would have good minds. But we are too good-hearted," said Stockholm resident Ulov Ekdahl, a 67-year-old commercial broker.

Joachim Kemiri, who was born in Sweden to a Tunisian father and a Swedish mother, said migrants and refugees had been arriving in too-large numbers.

"Too many of them have been coming in too fast. It's too much," the 29-year-old railway worker said.

The remains of the beer delivery truck that ploughed down Drottninggatan (Queen Street) in central Stockholm. ( Reuters: Maja Suslin/TT News Agency )

Sweden has long been known for its open-door policy toward migrants and refugees.

But after the Scandinavian country of 10 million took in a record 163,000 refugees in 2015 — the highest per-capita rate in Europe — Prime Minister Stefan Lofven conceded it could no longer cope with the influx.

At a press conference in late 2015, deputy prime minister Asa Romson broke into tears as she announced measures to deter asylum-seekers in a reversal of Sweden's welcoming policy toward people fleeing war and persecution.

She described it as "a terrible decision", admitting the proposals would make life even more precarious for refugees.

The suspect, a 39-year-old native of Uzbekistan, had been on authorities' radar previously ( Reuters )

But Mr Lofven made a point on Saturday of walking around Stockholm, including along Drottninggatan — the scene of the attack — chatting with people having coffee outside a cafe.

He said the aim of terrorism was to undermine democracy.

"But such a goal will never be achieved in Sweden," he said.

"We want — and I am convinced the Swedish people also want — to live a normal life. We are an open, democratic society and that is what we will remain."

ABC/wires