The stolen beer truck travelled for more than 500 metres along a main pedestrian street known as the Drottninggatan before it smashed into a crowd outside the upscale Ahlens department store about 3 p.m. Friday. He "showed interest for extremist organizations like IS", senior police officer Jonas Hysing told reporters.
The attack was the latest in a string of similar assaults involving vehicles in Europe, including in Nice in the south of France, London and Berlin.
The attack deeply shocked the usually tranquil Scandinavian nation, which prides itself on its openness and tolerance.
Police said that "there is nothing concrete to suggest further attacks", SVT reported. "Her body was in pieces", he said. Fear shall not reign. "Terror can not win", Stockholm Mayor Karin Wanngard told the crowd, saying terrorism would be defeated with "kindness and openness".
Flags flew at half-mast at public buildings across Stockholm.
The police arrested a 17-year-old and his mother from an address linked to the 39-year-old suspect from Uzbekistan. "The migration agency rejected it in June 2016 and also decided that he was to be expelled", Mr Hysing said.
According to Sky sources, the suspect was detained after police were notified of a man who was confused and acting strangely.
Four people were killed and 15 were injured when the lorry crashed into the front of a department store.
"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.
The mayor of Lembeek, in the Belgian city of Halle, says one of the four killed is a 31-year-old woman who was living in his town. It said two were in intensive care, four were seriously injured but the one child was not seriously hurt. A spokesperson for regional police told the New York Times that they believed the man to be the truck driver.
"There is nothing to indicate that we've got the wrong man".
In neighboring Norway early on Sunday, police set off a controlled explosion of a "bomb-like device" in central Oslo and took a suspect into custody.
A spokesman for German Chancellor Angela Merkel said: "We stand together against terror".
Witnesses described seeing the driver plowing through people without slowing down, before crashing into a department store.
A failed suicide bombing in December 2010 killed an attacker only a few hundred yards from the site of Friday's incident.