Suspicions about the arrested man have strengthened during the course of the investigation, Dan Eliasson, chief of the Swedish Police told a press briefing Saturday.
Police are investigating a "technical device" found in the vehicle used in the attack, he said.
"We confirm that we have found a device in the truck that doesn't belong there. We are now investigating its content," Eliasson said. "Whether this was a classic bomb or some sort of flammable device is now a matter for our analysis."
Prosecutor Hans Ihrman said the arrested man was a 39-year-old from Uzbekistan.
The attack Friday in the heart of Sweden's capital killed four people and injured about 15 more. Eight of the injured are still being treated in hospital, Eliasson said. Four of the injured, all adults, have serious injuries, Stockholm County Council said earlier. One child suffered minor injuries.
Earlier, Sweden's public broadcaster SVT reported that a bag of undetonated explosives had been found inside the truck, which was stolen minutes before the attack as it made a delivery at a restaurant. The attacker apparently suffered burns caused by the explosives, which did not detonate properly, SVT said.
A manhunt was launched immediately after the attack. The suspect was detained in the early hours of Saturday (Friday evening ET) in Marsta, a town about 30 minutes' drive north of Stockholm near the city's main international airport.
The man has featured in previous intelligence reports from last year, Anders Thornberg, chief of the Security Service told the briefing. However, the security services were not able to confirm that intelligence, so he was not on their radar, he said.
Their focus is now to prevent further attacks and investigate if there were other accomplices, Thornberg said. Security has been stepped up and border controls are in place.
Eliasson said investigators were examining the suspect's social media accounts and contacts, but declined to be drawn on whether he had connections to ISIS or other terror groups.
"We do not know whether there are further persons involved in this act or not," he said. "But we are not excluding that. We are still working on a very comprehensive approach to see whether there is any possibility that further individuals are involved."
Eliasson said he saw clear similarities to the attack in London
last month, in which a man plowed through pedestrians on Westminster Bridge before fatally stabbing a policeman outside the UK Parliament.
The prosecutor has until noon Tuesday to ask the courts to detain the suspect, said Karin Rosander, spokeswoman for Sweden's prosecutor's office.
'We are not afraid'
King Carl XVI Gustaf on Saturday expressed his condolences to the friends and families of the victims.
Speaking outside the royal palace in Stockholm, Gustaf said the nation was "shaken up" but urged people to seek peace.
"We have experienced other violence acts before and we have survived then, he said. "We will also survive now... Sweden is -- has been for a long time -- and shall continue to be a safe and peaceful country."
Drottninggatan (Queen Street) remained cordoned off Saturday morning, but the truck had been removed overnight from the building where it had been wedged. Heavily armed officers guarded the area and several police vans were present.
Scores of people came to lay flowers Saturday morning at a spot on Klarabergsgaten, near the crash site. The mood was subdued, with people seeming to be in quiet reflection.
Sweden's Crown Princess Victoria and her husband Prince Daniel visited the scene mid-morning. Princess Victoria, who was dressed in black and visibly moved to tears, was barely able to speak as she surveyed the area. "I feel an enormous sorrow and emptiness," she told reporters.
Prime Minister Stefan Lofven, who visited a short time later, said: "The goal is that we are supposed to be afraid, but Sweden has shown itself from its best side. That is a strength that no one can take away from us."
Police consoled one crying man nearby as he walked up with a bouquet. Several fathers brought their children down to the barrier and explained gently what happened.
Habib Jlassi, a 29-year-old night bus driver who was there with his three-year-old daughter, told CNN it was important to pay tribute to those who lost their lives.
"I've come here to show that we are not afraid," said Jlassi, who lives in Stockholm and whose eyes appeared red from crying. "We are stronger than them."
Around the corner, in a square outside Stockholm Concert Hall, a flower, fruit and vegetable market was being set up. Some people bought flowers there before walking over to leave them by the cordon.
Debris still littered the ground around the crash scene and workers in neon suits carried items to a waiting skip.
Some shops opened Saturday morning on streets around Drottninggatan as the city center began to return to normal.