Police have not named the suspected driver of the truck, whom they arrested on Friday evening, but authorities said he was known to Sweden's intelligence service for undisclosed reasons.
Update 1pm: The Stockholm truck attack suspect was an asylum seeker from Uzbekistan who had his application for Swedish residency rejected and was being sought for deportation, police said. "We know that he showed sympathies for extremist organisations, among them ISIS", national head of police operations Jonas Hysing told reporters.
Four people were killed in the horrific assault and 10, including a child, remain in hospital.
On Sunday, people gathered in central Stockholm for a "Lovefest" vigil against terrorism, and laid flowers outside the Ahlens shop in tribute.
A truck rammed into people on a pedestrian street in Stockholm before crashing into a department store on April 7, killing and injuring many civilians.
The attack had other nearby nations on high alert, including Norway, where a large section of the capital city of Oslo was cordoned off last night after authorities found what they described as a "bomb-like" device near a busy subway station.
In 2014, the suspect had applied for residency but it was rejected in December previous year, another police spokesman, Jan Evensson, told reporters.
Sweden's public broadcaster SVT reported that a bag of explosives was found inside the truck on the driver's seat and the suspected attacker suffered burns from a partial detonation.
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.
The more than a dozen people who had been hospitalized due to the truck attack, six have been cleared for release and are now at home. A technical device which was "not supposed to be there" had been placed inside the glove compartment, says Stockholm police chief Dan Eliassson.
The suspect in a truck-ramming attack in Stockholm, Sweden on Saturday, had reportedly hijacked a beer truck several blocks earlier.
Footage has emerged showing the lorry used in the Stockholm terror attack hurtling down a pedestrianised street as shoppers run for cover.
However, Reuters news agency said the person was arrested on "a lower degree of suspicion" than the first suspect.