"We know that he showed interest for extremist organizations like IS", Hysing said. In June 2016, Swedish Migration Board rejected his application and in December he was given a deadline of four weeks to leave the country.
Currently, there are three men suspected of the involvement in the Friday attack under police arrest, of whom two were taken on Friday and the other on Sunday.
The investigation led to the arrest of another suspect in the case, Helga Hullmann, a judge at the Stockholm District Court, told Reuters.
Stockholm officials, meanwhile, moved thousands of flowers at a makeshift memorial to a nearby square after a fence outside the department store was overwhelmed with tributes and threatened to collapse.
The main suspect, a 39-year-old Uzbek man, is in custody following the attack.
Crown Princess Victoria of Sweden is paying her respects to the victims of Friday's deadly attack in Stockholm.
Eliasson said there were "clear similarities" to an attack last month in London in which six people died, including the assailant who drove a hired vehicle into pedestrians on a bridge.
On Friday, the truck crashed into a crowd of pedestrians in the center of Stockholm.
He had been seen only as a "marginal character", National Police Commissioner Dan Eliasson said.
He said it "could be a bomb or an incendiary object, we are still investigating it". Mr Hysing said "the evidence looks very strong" that the Uzbek man was the driver of the hijacked truck.
Fifteen people were treated in hospitals for injuries they suffered in the attack. Four of them are in "serious condition", health authorities told AFP.
A second person was also arrested, "suspected of terror crimes through murder", according to Sweden's prosecutor's office.
There was no immediate claim of responsibility for the attack.
The suspect was detained Friday in Marsta, about 25 miles north of Stockholm near the city's main worldwide airport.
Sweden is statistically one of the safest nations in the world, and the country's prime minister and Stockholm's mayor have both vowed that the attack will not damage the country and city's open society.
Swedish Prime Minister Stefan Lofven said one person has been arrested in connection to the apparent "terror attack".
Soderberg said it was important that caregivers continue to give "long-term psychological support to those who need it".