The Swedish police arrested a 39-year-old Uzbek man on Friday evening as the suspected driver of the truck, and the officials are now questioning seven more people.
"We know he has been sympathetic to extremist organizations", said Jonas Hysing of Sweden's national police. "The Migration Agency rejected it in June 2016 and also decided that he was to be deported", Hysing said.
The suspect ignored the notification and his case was transferred to the police to execute the deportation order, but the man "went underground".
A Swedish prosecutor says a person has been formally identified as a suspect "of terrorist offenses by murder" by driving a hijacked truck into a crowd of pedestrians, killing four people and wounding 15 others.
Many Swedes were back at work for the first time since the attack, while the department store into which the truck slammed had already reopened. He said the suspect was not on any current list of people being monitored.
Meanwhile, Swedish police arrested a second suspect accused of involvement in the attack.
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Officials said the man they now have in custody matches the description of a person of interest, but have not confirmed if he was the driver. Head of the National Police, Dan Eliasson, said that the suspect had been known to the police.
One police officer involved in the operation, Abdallah Ahmed, said he and his colleagues were "exposed to stone throwing" as they entered the district of Tensta on Saturday evening.
Swedish citizens appeared largely shocked by the attack, with many bringing flowers to the site, and some bringing their children to teach them they must pay tribute to the victims, worldwide media reported.
"Sweden shall come out stronger from this, and we get through this together", she told the press and mourners gathered near the department store.
Four people were killed in the horrific assault and 10, including a child, remain in hospital. And though it's unclear whether the suspect was a Swedish citizen or how long he'd been in the country, the attack had some city residents re-evaluating the country's stance on immigration.
Police said they were unable to confirm local television reports of explosives being found inside the truck. In December 2010, a suicide bomber blew himself up, also on the Drottninggatan street, slightly injuring several passersby.