The suspected Stockholm truck attacker had shown interest in extremist groups and had his permanent residency application rejected in June 2016, Swedish police said.
"We know that he showed interest for extremist organisations like IS. He was applying for a residence permit that was rejected in 2016," police chief Jonas Hysing told reporters.
Hysing added that two Swedes, one Briton and a Belgian were killed in the attack.
Stockholm district court judge Helga Hullman also told AFP that a second suspect has been formally placed under arrest in connection with the attack.
"I can confirm that a second person has been arrested," Hullman said.
It comes as thousands of people gathered in central Stockholm for a vigil against terrorism.
Shocked by Friday's attack that left four dead and 15 injured, Stockholmers mobilised on Facebook to organise a vigil at the Sergels Torg plaza near where the truck rammed into shoppers.
The main suspect, a 39-year-old Uzbek man, is in custody following the attack.
Sweden has been trying to get back on its feet after what authorities termed a terror attack, the motive for which was still unknown.
The method however was similar to previous attacks using vehicles in Nice, Berlin and London, all of them claimed by the Islamic State (IS) group.
There has been no immediate claim of responsibility for the Stockholm attack - the third in Europe in two weeks, coming on the heels of the car and knife assault outside London's parliament and the Saint Petersburg metro bombing.
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.
The man is suspected of speeding a stolen beer truck several hundred metres down the bustling pedestrian street Drottninggatan in the heart of Stockholm.
The vehicle mowed down shoppers before slamming into the facade of the busy Ahlens department store.
"There is nothing to indicate that we've got the wrong man. On the contrary, the suspicions have strengthened," Swedish police chief Dan Eliasson said Saturday.
He said police found a suspect device in the cab of the truck.
"A technical examination is ongoing, we can't go into what it is right now... whether it's a bomb or a flammable device."
Six people were taken into custody for interrogation on between Saturday and Sunday in several areas across Stockholm, police said, without adding further details.
Become 'even more open'
Ten people, including one child, are still in hospital. Four of them are in "serious condition", health authorities told AFP.
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Oslo police detonate 'bomb-like device'
In neighbouring Norway on Sunday, police said they had destroyed a suspect "bomb-like" device in the capital Oslo and made one arrest.
Friday's attack in Stockholm deeply shocked the usually tranquil Scandinavian nation, which prides itself on its openness and tolerance.
All day Saturday, crowds milled behind the security fences blocking off the scene of the attack, laying flowers on the ground or poking them into the fence.
Several police cars parked near the scene were also covered in flowers by Swedes, who widely praised the emergency crews' speedy response to the attack.
"Maybe something good will come of this," Inger Morstedt, 75, told AFP, expressing hope that her fellow Swedes would become "even more open and welcoming".
"In some ways it's unreal," said 40-year-old Johan.
"I've come here to honour the victims and the society in which we live."
Flags flew at half-mast at public buildings across Stockholm on Saturday.
WATCH: People flee as the truck speeds down one of Stockholm's busiest shopping streets
Prime Minister Stefan Lofven, who beefed up border controls on Friday after the attack, announced a national minute of silence to be held in honour of the victims on Monday at noon (1000 GMT).
"Today, all of Sweden is in mourning, but we're going to get through this together," he told reporters on Saturday after laying a bouquet outside the Ahlens department store.
King Carl XVI Gustaf, who returned to Stockholm on Saturday after cutting short a visit to Brazil, also addressed the nation outside the palace.
"The consideration people are showing each other shows the strength of our society," he said.
"There are so many of us who want to help, many more than those who want to hurt us."
Friday's attack was the second terror attack in Stockholm.
In December 2010, a suicide bomber blew himself up, also on the Drottninggatan street, lightly injuring several passersby.
