"We know he has been sympathetic to extremist organizations", said Jonas Hysing of Sweden's national police.
A woman and an 11-year-old girl from Sweden were killed in the attack, as well as a 41-year-old Briton who worked for Spotify and a Belgian woman.
Swedish police have identified a 39-year-old man originally from Uzbekistan as the suspected driver. "In February 2017, the case was handed over to the police to carry out the order, since the person had gone underground".
On Saturday, the police chief, Dan Eliasson, said: "We have found something in the truck in the driver's compartment, a technical device that should not be there".
A man arrested after a stolen truck plowed into pedestrians on a busy Stockholm street is being held on "suspicion of terrorist crimes through murder", a government official said Saturday.
The police team also included a Muslim officer, Abdallah Ahmed, said, "To terror I want to say one thing, in pure Swedish: go to hell".
Thousands of people have come to the site, including the prime minister and members of Sweden's royal family, to lay flowers and light candles in memory of the truck attack victims.
Swedish police said they had not ruled out that the attacker could have had others helping him.
The lorry-hijacked from outside a nearby restaurant-crashed into the front of Ahlens department store in the middle of the afternoon on Drottninggatan (Queen Street), one of the city's main pedestrian thoroughfares.
Sweden's health service said 10 people were still hospitalized for wounds from the attack and four of them were seriously injured.
"We do not comment on what we have seized", said Bystrom, declining to confirm or deny media reports that a bag with some kind of explosives had been found in truck.
Last month, a man rammed into crowds on Westminster Bridge in London, killing three people before stabbing a policeman to death outside Parliament.
Swedish Prime Minister Stefan Lofven, who beefed up border controls last Friday after the attack, announced a national minute of silence to be observed in honour of the victims at noon today.
"Canada grieves with our Swedish friends in this hard time and we offer all possible assistance", the prime minister said in his statement on Friday.
In a statement on its Facebook page, the Ahlens department store apologized "for a bad decision".
The department store has said it will reopen on Monday (local time), but on Sunday it was forced to apologise for an announcement it made saying it would be selling damaged goods at a "reduced price".