A national manhunt was under way for those responsible and one person was arrested in connection with the attack.
According to SVT, the man was arrested in the northern suburb Hjulsta and he is said to be connected to the man who was arrested earlier on Friday.
Neutral Sweden has not fought a war in more than 200 years, but its military has taken part in United Nations peacekeeping missions in a number of conflict zones in recent years, including Iraq, Mali and Afghanistan.
The driver of the truck - which according to Daily Mail was stolen from a brewery - is still at large.
"We are still trying to determine who the attacker was, if the attack was carried by one or more people, and the number of injured".
Detectives refused to confirm whether the man arrested was the same one shown in the image.
Photos taken at the scene show the vehicle was a truck belonging to beermaker Spendrups, which said that its truck had been carjacked earlier in the day.
Last month, Khalid Masood, a 52-year-old convert to Islam known to British security services, drove a vehicle at high speed into pedestrians on London's Westminster Bridge before launching a frenzied knife attack on a policeman guarding the parliament building. He was immediately returning to the capital.Many police and emergency services personnel were at the scene, said a Reuters witness who saw policemen put what appeared to be two bodies into body bags. "Everything points to the fact that this is a terrorist attack", Prime Minister Stefan Lofven told reporters during a visit to western Sweden.
The bloodshed bore hallmarks to attacks seen in Nice and Berlin past year, where a truck rammed pedestrians, and M uslim convert Khalid Masood's attack on Westminster Bridge in London. He says "I looked out of the store and saw a big truck".
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Police said it's unclear whether this was an isolated attack or if it's related to any other incidents. I was in a nearby store in Drottninggatan Street when I heard women speaking in very high tone. "Heavy weapons, civilian police, etc", he said. People were lying on the ground.
One witness identified only as Dimitris told the Aftonbladet that the truck came "out of nowhere".
"There was a sense of panic, people standing by their loved ones, but also people running away".
Another section of Drottninggatan was also the scene of Sweden's only other terror attack, in 2010, when a suicide bomber blew himself up, killing himself and slightly injuring several others.
Abdulwahab rigged an Audi auto with explosives in the hope that the blast would drive people to Drottninggatan, a busy shopping street, where he was waiting to set off two more devices strapped to his chest and back.
Prosecutors have given detectives more time to question the suspect in custody, Stockholm police said.
Sweden's national police chief told the press they have the truck, and are in contact with the truck's normal driver.
The attack was the latest to hit the Nordic region after shootings in Copenhagen, Denmark, in 2015 that killed three people, and the 2011 bombing and shooting by far-right extremist Anders Behring Breivik that killed 77 people in Norway.
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